Characterisation of Nearby Ultracool Dwarf Candidates with OSIRIS/GTC: First Detection of Balmer Line Emission from the Dwarf Carbon Star LSR J2105+2514

Abstract

Based on low-resolution OSIRIS/GTC optical spectra, we assign spectral classes to 38 poorly studied ultracool/brown dwarf candidates from the 2MASS database. For almost all of the targets, this is the first optical spectral classification. For the dwarfs showing H$\alpha$ emission, we calculate the ratio of H$\alpha$ to bolometric luminosity, which is the most common characteristic of magnetic activity in cool stars. For the others, we give 3$\sigma$ upper limits. We also include estimates of the effective temperatures and log g and distances from Gaia based on a comparison with models. For one of our targets—LSR J2105+2514, previously classified as a dwarf carbon star—we confirm this classification and report H$\alpha$ and H$\beta$ line emission in the spectrum for the first time. Dwarf carbon stars (dC) are low-mass main sequence stars that have undergone mass-transfer binary evolution. The Balmer line emission from these objects most likely indicates coronal activity of the dwarf, which in turn may be due to either intrinsic magnetic activity or spin-up from accretion or tidal locking.

1. Introduction

The study of nearby stars is paramount for addressing a wide range of astrophysical problems, including star formation, evolution, the stellar luminosity function, galactic dynamics, and searches for planetary systems, including potentially habitable ones [1]. The nearest stars, being the brightest examples of their types, provide much of our understanding of such stars’ nature, and thus, the rudimentary framework of stellar astronomy is built on direct measurements of the fundamental parameters of stars in the solar neighbourhood. One of the most important products of nearby star research is a volume-limited sample of nearby stars. All-inclusive volume-limited samples of stars will allow us to answer large numbers of questions about the formation, kinematics, stellar mass fraction, and metallicity distributions of stars. Moreover, a complete census of the solar neighbourhood (including spectral classification and magnetic activity indicators) is highly desirable for both earth- and space-based planetary searches.

M dwarfs comprise about 70% of the total stellar population [2], and despite great advances in recent years, a great many candidates for late-type cool dwarfs have yet to be properly identified and studied. In addition, M dwarfs were suggested to be viable candidates for detecting planets in habitable zones [3]. On one hand, they comprise the largest population of stars, and on the other, their lower luminosities and smaller masses make the detection of potential exoplanets around them more promising (by the RV method and/or by the transit method [4,5]). In recent years, there have been quite a few searches and surveys dedicated to just this task (e.g., HARPS [6], CARMENES [7], Borucki et al. [8] and Hsu et al. [9] using the Kepler mission, Eschen and Kunimoto [10] using the TESS mission, and most recently Guzman Caloca et al. [11] with JWST).

Intrinsic stellar activity, however, can cause additional variations of the same order of magnitude as the signal induced by a planet [12]. On the other hand, stellar activity can prove either detrimental or beneficial to the emergence of life on potentially habitable planets [13,14,15]. It is therefore crucial to quantify the activity of the target stars in terms of strength and variability. The most commonly used parameters of magnetic activity in cool dwarfs are the ratios of chromospheric H$\alpha$, coronal X-ray, and radio luminosities to the bolometric luminosity. For active M dwarf stars (dMe stars), these characteristics are found to exhibit a certain behaviour with spectral type as well as relate closely to each other [16]. H$\alpha$ activity levels, in particular, are seen to increase towards later types due to longer spin-down timescales, reaching a peak at spectral type∼M9-L0 [17].

Here we present the results from low-resolution GTC/OSIRIS observations of 38 poorly studied high-proper-motion (i.e., expected to be nearby) candidates for ultracool dwarfs with the aim of obtaining optical spectral classification and determining the levels of their H$\alpha$ activity for the first time. These were combined with data from Gaia DR1, DR2, and DR3 [18,19,20,21] to acquire accurate distances. We also calculated bolometric magnitudes and luminosities. Atmospheric parameters were obtained by comparison of the observed spectra with BT-Settl (CIFIST) models [22,23,24].

In Section 2, we describe the target selection, observations, data reduction, and archival data. Section 3 presents the spectral classification and estimates of effective temperature and log g, while Section 4 presents estimates of chromospheric activity. Section 5 is dedicated to one interesting object, 2MASS J21051653+2514486, and Section 6 contains a brief discussion.

2. Target Selection, Observations, and Data Reduction

2.1. Target Selection

The selection criteria for the late-M candidates are similar to those described in Metodieva et al. [25], including the following:

• Colours. They are based on 2MASS colour indices: 1.0 < (J − K) < 1.3 mag; 0.25 < (H − K) < 0.60; 0.60–1.33 × (H − K) + 3.00 × (H − K)² < (J − H); (J − H) < 0.90–1.33 × (H − K) + 3.00 × (H − K)². In addition, the accuracy of the colours should be better than 0.15 mag. The calibration of 2MASS colours for main sequence star–spectral type is based on dwarfs taken from the DwarfArchives [26]. The colour–colour diagram for the selected possible late-M dwarfs is shown in Figure 1.
• Proper motions and observational constraints. To distinguish neighbouring from more distant dwarfs or giants, only objects with proper motions > 0.3”/yr were considered. Proper motions were determined using at least two epochs of observations: position from the 2MASS catalogue [27] and from WISE [28]. In some cases, there were third epoch data from SDSS [29]. The time base of the observations varies between 9 and 13 years. Our project began before the first published Gaia data, so we used those additional positions of our targets only to improve determined proper motions.
• Brightness. To satisfy the observational requirements, we constrained magnitudes to 10 < J < 16, since dwarfs with J < 9 are likely to have already been studied by, e.g., Lépine et al. [30], and dwarfs with J > 16 mag are too faint to obtain enough signal in imperfect weather conditions and short exposures even with GTC. Following these criteria and observational constraints, ∼40 targets were selected, and we obtained good quality spectroscopic data for 38 candidates (

  Table 1. Observation log for the candidate dwarfs. Columns are 2MASS ID, RA and Dec, proper motions, date of observation, grism, and exposure time.

  2MASS ID	RA [J2000]	Dec [J2000]	pm [mas/yr]	Observ. Date	Grism	Exposure [s]
  J00050184+0217120	001.257667	+02.286683	327.8, 215.0	23 November 2016	R	2 × 700
  J00271090-1813097	006.795440	−18.219379	398.6, −97.4	27 November 2016	R	2 × 950
  J01013443+0336216	015.393459	+03.606020	604.9, −565.7	2 September 2016	R	4 × 1400
  J01135973-1408174	018.498882	−14.138179	280.2, −367.0	7 December 2016	R	2 × 1400
  J01304280-0211294	022.678335	−02.191508	498.4, 28.6	14 September 2016	R	2 × 500
  J02133713-1343228	33.404743	−13.723012	−359.2, −522.0	24 September 2016	R	2 × 1400
  J03110650+0417360	47.777122	+04.293337	402.2, −49.3	23 September 2016	R	2 × 1400
  J03184214+0828002	105.763702	+07.183564	142.8, −411.5	25 November 2016	R	2 × 500
  J00050184+0217120	001.257667	+02.286683	327.8, 215.0	23 November 2016	R	2 × 700
  J00271090-1813097	006.795440	−18.219379	398.6, −97.4	27 November 2016	R	2 × 950
  J01013443+0336216	015.393459	+03.606020	604.9, −565.7	2 September 2016	R	4 × 1400
  J01135973-1408174	018.498882	−14.138179	280.2, −367.0	7 December 2016	R	2 × 1400
  J01304280-0211294	022.678335	−02.191508	498.4, 28.6	14 September 2016	R	2 × 500
  J02133713-1343228	33.404743	−13.723012	−359.2, −522.0	24 September 2016	R	2 × 1400
  J03110650+0417360	47.777122	+04.293337	402.2, −49.3	23 September 2016	R	2 × 1400
  J03184214+0828002	105.763702	+07.183564	142.8, −411.5	25 November 2016	R	2 × 500
  J03421129+4629365	055.547083	+46.493492	353.3, −390.8	28 September 2016	R	2 × 1400
  J04134574+3709087	63.440596	+37.152443	619.4, −225.9	28 September 2016	R	2 × 1400
  J04204796+5624202	65.199861	+56.405624	591.5, −134.6	23 September 2016	R	2 × 500
  J05085506+3319272	77.229430	+33.324226	228.4, −655.3	21 November 2016	R	2 × 1400
  J05331650+3740280	83.318754	+37.674458	155.8, −588.8	30 September 2016	R	2 × 500
  J05385671-0808296	84.736321	−08.141577	443.6, −460.7	1 October 2016	R	2 × 500
  J06142970+3833415	093.623749	+38.561550	83.9, −395.8	26 September 2016	R	2 × 950
  J07011762+2401319	105.323456	+24.025532	208.4, −347.2	21 November 2016	R	2 × 500
  J07030328+0711008	105.763702	+07.183564	142.8, −411.5	25 November 2016	R	2 × 500
  J07561916+6234493	119.079857	+62.580383	−154.7, −370.0	9 October 2016	R	1 × 1400
  J08175223+5246117	124.467644	+52.769924	−140.4, −377.6	27 September 2016	R	2 × 1400
  J08330310+3706083	128.262955	+37.102322	−272.3, −419.3	8 October 2016	R	2 × 500
  J08440874+7101007	131.036433	+71.016869	−282.8, −338.7	21 November 2016	R	2 × 1400
  J10365971+5932068	159.248795	+59.535236	−194.3, −354.0	1 December 2016	R	2 × 1400
  J11332017+1039295	173.334072	+10.658220	−381.6, −71.5	10 December 2016	R	2 × 950
  J11553775+0922227	178.907305	+09.372981	−371.4, −68.6	27 November 2016	R	2 × 950
  J15495293+0151167	237.470551	+01.854645	−246.5, −377.4	19 January 2017	R	2 × 1400
  J17373855+4705511	264.410645	+47.097553	52.1, −510.6	30 September 2016	R	2 × 1400
  J19383880+6010182	294.661682	+60.171726	−68.6, −450.0	11 October 2016	R	2 × 500
  J21051653+2514486	316.318903	+25.246847	274.8, −477.9	10 October 2016	R	2 × 600
  				6 December 2016	B	2 × 900
  				7 December 2016	R	2 × 600
  J21161051+0341294	319.043823	+03.691509	408.9, −245.3	8 October 2016	R	2 × 950
  J21265788+2531080	321.741180	+25.518911	−120.6, −666.7	1 October 2016	R	2 × 500
  J21580211+0409197	329.508789	+04.155493	99.0, −427.0	25 September 2016	R	2 × 950
  J21580671+5836379	329.527978	+58.610531	609.2, 335.6	1 October 2016	R	2 × 500
  J22240946-1852387	336.039429	−18.877436	251.8, −341.9	20 November 2016	R	2 × 1400
  J22292894-0444005	337.370575	−04.733497	445.6, 207.9	20 November 2016	R	2 × 1400
  J22512440+2952452	342.851685	+29.879230	498.6, 186.7	1 October 2016	R	2 × 1400
  J23100915+3230083	347.538147	+32.502327	433.4, 84.4	20 November 2016	R	2 × 1400
  J23274947+0450583	351.956162	+04.849550	290.5, 149.6	28 November 2016	R	2 × 1400
  J23333910+3925057	353.412903	+39.418259	400.9, −73.1	2 October 2016	R	2 × 1400

  ).

2.2. Observations and Data Reduction

Low-resolution spectroscopy was carried out with the Optical System for Imaging and Low Resolution Integrated Spectroscopy (OSIRIS) tunable imager and spectrograph [31,32] at the 10.4 m Gran Telescopio Canarias (GTC), located at the Observatorio Roque de los Muchachos in La Palma, Canary Islands, Spain. The heart of OSIRIS is a mosaic of two 4k × 2k e2v CCD44–82 detectors that gives an unvignetted field of view of 7.8 × 7.8 arcmin² with a plate scale of 0.127 arcsec pix⁻¹. However, to increase the signal-to-noise ratio of our observations, we chose the standard operation mode of the instrument, which is a 2 × 2-binning mode with a readout speed of 100 kHz. The spectra were obtained with the OSIRIS R1000R (red) grism. It has a wavelength range of 5100–10,000 Å, centred on 7510 Å, with a resolution of 1122 (∼2.62 Å/px) and maximum quantum efficiency of 65%. For one object we also used the R1000B (blue) grism, which has a wavelength range of 3630–7500 Å, centred on 5510 Å, with a resolution of 1018 (∼2.12 Å/px) and maximum quantum efficiency of 65%. We used the 1.23 arcsec-width slit, oriented at the parallactic angle to minimise losses due to atmospheric dispersion. The resulting resolution, measured on arc lines, was R∼700 in the approximate 3500–8000 Å spectral range.

The observations were performed in service mode within the GTC “filler” programme GTC52-16B on different nights in observational season 2016B. Filler programs are utilised to improve the observing efficiency and overall productivity of the Observatory under sub-optimal observing conditions. Our program was designed and planned taking these specifics into account, capitalising on the unique light-gathering capabilities of the GTC. The standard OSIRIS queue-observing strategy was followed. The instrument was mounted on the Nasmyth B GTC focal station and granted excellent stability. Calibration frames (arcs and flats) were acquired once per night, covering all utilised observing configurations. The same applies for spectrophotometric standards. Telluric standards were not included in the standard GTC calibration queue. To avoid the influence of the most prominent tellurics, datasets were restricted to 9200 Å. Below these wavelengths, tellurics are not removed from the spectra. A detailed observational log is presented in Table 1.

Data reduction for all spectra was performed using the software package IDL (Interactive Data Language) and its astronomical libraries [33]. The code was written by us especially for OSIRIS spectra [25]. The data reduction followed standard procedures, and flux calibration was made using standard stars with well-known spectral energy distributions. The resultant spectra are shown in Figure A1 and Figure A2.

2.3. Archival Data

All archival data used in the present work were retrieved with the Tool for OPerations on Catalogs And Tables (TOPCAT; Taylor [34]), utilising the object identifiers according to the 2MASS nomenclature. Taking into account the high proper motions of the objects studied, an iterative procedure with gradually increasing search radii (1" increment) was applied. In certain ambiguous cases with multiple objects retrieved within the search area, the proper motions as measured by Gaia DRs were used as an extra proxy to identify the proper object. Gaia, WISE, and 2MASS information was recovered for each dwarf star in our sample (

Table A1. Gaia DR3 photometry data. First two columns contain the 2MASS and Gaia identifications of the objects. Columns three, four, and five give the Gaia DR3 G, BP, and RP magnitudes, respectively. Column six shows the search radius applied from the 2MASS position of the object in order to recover it in the Gaia database.

2MASS ID	Gaia ID	${\mathit{G}}_{\mathbf{DR}3}$	${\mathbf{BP}}_{\mathbf{DR}3}$	${\mathbf{RP}}_{\mathbf{DR}3}$	r
		[mag.]	[mag.]	[mag.]	["]
J00050184+0217120	2739077223651402240	$17.836\pm 0.003$	$20.77\pm 0.13$	$16.282\pm 0.007$	7
J00271090-1813097	2366831549112880000	$17.571\pm 0.003$	$20.60\pm 0.09$	$16.052\pm 0.005$	7
J01013443+0336216	2551477793805008256	$19.482\pm 0.005$	$21.26\pm 0.17$	$17.95\pm 0.02$	14
J01135973-1408174	2456022519312921472	$18.763\pm 0.004$	$21.32\pm 0.16$	$17.173\pm 0.010$	8
J01304280-0211294	2485433257060541184	$17.300\pm 0.003$	$20.51\pm 0.07$	$15.743\pm 0.005$	9
J02133713-1343228	5149965026866229760	$19.116\pm 0.004$	$21.24\pm 0.15$	$17.550\pm 0.011$	9
J03110650+0417360	2781562554898432	$18.558\pm 0.004$	$21.46\pm 0.20$	$16.982\pm 0.010$	7
J03184214+0828002	11064763467877504	$18.039\pm 0.004$	$20.85\pm 0.14$	$16.493\pm 0.007$	5
J03421129+4629365	247887099802005504	$18.109\pm 0.003$	$21.03\pm 0.11$	$16.542\pm 0.007$	8
J04134574+3709087	177700805837431296	$18.620\pm 0.003$	$21.09\pm 0.16$	$17.015\pm 0.009$	8
J04204796+5624202	276870604275149568	$16.980\pm 0.003$	$20.00\pm 0.05$	$15.476\pm 0.004$	9
J05085506+3319272	181724125038647040	$19.141\pm 0.004$	$21.7\pm 0.2$	$17.598\pm 0.012$	11
J05331650+3740280	190314025273114752	$16.482\pm 0.003$	$19.39\pm 0.04$	$15.012\pm 0.004$	8
J05385671-0808296	3015405730029371392	$17.061\pm 0.003$	$20.12\pm 0.07$	$15.549\pm 0.005$	9
J06142970+3833415	956200977271782144	$18.143\pm 0.004$	$20.85\pm 0.12$	$16.555\pm 0.007$	7
J07011762+2401319	3380600854974394880	$16.742\pm 0.003$	$19.49\pm 0.05$	$15.262\pm 0.005$	7
J07030328+0711008	3153877399801800576	$16.869\pm 0.003$	$19.98\pm 0.07$	$15.363\pm 0.004$	7
J07561916+6234493	1085224509260180736	$18.339\pm 0.003$	$21.22\pm 0.10$	$16.790\pm 0.005$	7
J08175223+5246117	1031554907167231872	$18.879\pm 0.004$	$21.39\pm 0.22$	$17.300\pm 0.010$	7
J08330310+3706083	910087150363664000	$16.438\pm 0.003$	$19.26\pm 0.05$	$14.949\pm 0.005$	11
J08440874+7101007	1121588966563321856	$18.422\pm 0.003$	$21.17\pm 0.13$	$16.958\pm 0.008$	8
J10365971+5932068	1047702987768668928	$17.850\pm 0.003$	$20.58\pm 0.10$	$16.357\pm 0.006$	7
J11332017+1039295	3916235366147580288	$17.663\pm 0.003$	$20.97\pm 0.15$	$16.125\pm 0.006$	7
J11553775+0922227	3912151844386051584	$17.648\pm 0.003$	$20.27\pm 0.05$	$16.184\pm 0.005$	6
J15495293+0151167	4423184031270272896	$18.489\pm 0.003$	$20.94\pm 0.12$	$16.924\pm 0.010$	8
J17373855+4705511	1363482108789712000	$19.237\pm 0.004$	$21.04\pm 0.15$	$17.676\pm 0.018$	8
J19383880+6010182	2238983395052535936	$18.682\pm 0.004$	$20.8\pm 0.2$	$17.136\pm 0.012$	8
J21051653+2514486	1841350340628210176	$16.697\pm 0.003$	$17.657\pm 0.008$	$15.746\pm 0.004$	10
J21161051+0341294	1731827128389211520	$18.406\pm 0.003$	$21.09\pm 0.15$	$16.809\pm 0.010$	8
J21265788+2531080	1798588409277193984	$17.002\pm 0.003$	$20.56\pm 0.09$	$15.446\pm 0.005$	11
J21580211+0409197	2695875380274977280	$17.804\pm 0.003$	$20.76\pm 0.18$	$16.249\pm 0.009$	7
J21580671+5836379	2199435611080216704	$17.470\pm 0.003$	$20.80\pm 0.07$	$15.893\pm 0.005$	11
J22240946-1852387	6821768338025389440	$18.797\pm 0.004$	$21.42\pm 0.24$	$17.216\pm 0.014$	7
J22292894-0444005	2626353401784545280	$18.401\pm 0.004$	$21.0\pm 0.3$	$16.863\pm 0.014$	8
J22512440+2952452	1887751380587578112	$17.643\pm 0.003$	$20.40\pm 0.07$	$16.156\pm 0.005$	9
J23100915+3230083	1910773333005191680	$18.295\pm 0.003$	$21.08\pm 0.10$	$16.792\pm 0.009$	7
J23274947+0450583	2660317419045705344	$19.485\pm 0.008$	$20.92\pm 0.23$	$17.98\pm 0.02$	8
J23333910+3925057	1920130348756557952	$18.661\pm 0.003$	$21.21\pm 0.19$	$17.098\pm 0.009$	7

and

Table A2. 2MASS NIR and WISE MIR photometry data. The first column contains the shortened 2MASS identification of the objects. Columns two to four show the 2MASS J, H, and Ks magnitudes, followed by the 2MASS photometry quality flag in column five. Columns six to nine contain the recovered W1, W2, W3, and W4 WISE magnitudes.

2MASS ID	J	H	Ks	Flag	W1	W2	W3	W4
	[mag.]	[mag.]	[mag.]		[mag.]	[mag.]	[mag.]	[mag.]
J0005+0217	$13.69\pm 0.03$	$13.06\pm 0.04$	$12.68\pm 0.02$	AAA	$12.45\pm 0.02$	$12.23\pm 0.02$	$11.3\pm 0.2$	$9.0$
J0027-1813	$13.45\pm 0.02$	$12.85\pm 0.03$	$12.43\pm 0.03$	AAA	$12.25\pm 0.02$	$12.02\pm 0.02$	$11.9\pm 0.3$	$8.7$
J0101+0336	$15.42\pm 0.05$	$14.65\pm 0.07$	$14.30\pm 0.07$	AAA	$14.21\pm 0.03$	$13.94\pm 0.04$	$12.3\pm 0.4$	$8.6$
J0113-1408	$14.20\pm 0.03$	$13.49\pm 0.03$	$13.12\pm 0.03$	AAA	$12.78\pm 0.02$	$12.48\pm 0.03$	$11.6\pm 0.3$	$8.7$
J0130-0211	$13.03\pm 0.02$	$12.37\pm 0.03$	$12.02\pm 0.03$	AAA	$11.79\pm 0.02$	$11.56\pm 0.02$	$11.24\pm 0.12$	$9.1$
J0213-1343	$14.48\pm 0.03$	$13.80\pm 0.04$	$13.34\pm 0.04$	AAA	$13.06\pm 0.02$	$12.77\pm 0.03$	$12.1\pm 0.2$	$9.3$
J0311+0417	$14.23\pm 0.03$	$13.57\pm 0.03$	$13.16\pm 0.03$	AAA	$12.88\pm 0.02$	$12.62\pm 0.02$	$11.8$	$8.9$
J0318+0828	$13.79\pm 0.03$	$13.09\pm 0.03$	$12.69\pm 0.02$	AAA	$12.47\pm 0.02$	$12.27\pm 0.02$	$11.5\pm 0.3$	$8.8$
J0342+4629	$13.79\pm 0.03$	$13.10\pm 0.03$	$12.68\pm 0.03$	AAA	$12.39\pm 0.02$	$12.15\pm 0.02$	$11.7\pm 0.3$	$8.4$
J0413+3709	$13.79\pm 0.03$	$12.99\pm 0.04$	$12.50\pm 0.02$	AAA	$12.11\pm 0.02$	$11.84\pm 0.02$	$11.5\pm 0.3$	$8.3$
J0420+5624	$12.97\pm 0.02$	$12.31\pm 0.02$	$11.94\pm 0.02$	AAA	$11.69\pm 0.02$	$11.49\pm 0.02$	$11.07\pm 0.12$	$8.9$
J0508+3319	$14.22\pm 0.03$	$13.24\pm 0.04$	$12.62\pm 0.03$	AAA	$12.16\pm 0.03$	$11.89\pm 0.02$	$12.1\pm 0.4$	$8.8$
J0533+3740	$12.54\pm 0.02$	$11.90\pm 0.02$	$11.52\pm 0.02$	AAA	$11.29\pm 0.02$	$11.11\pm 0.02$	$11.2\pm 0.2$	$8.9$
J0538-0808	$13.08\pm 0.03$	$12.39\pm 0.03$	$12.01\pm 0.03$	AAA	$11.80\pm 0.02$	$11.60\pm 0.02$	$11.6\pm 0.2$	$8.8$
J0614+3833	$13.52\pm 0.02$	$12.75\pm 0.03$	$12.25\pm 0.02$	AAA	$11.85\pm 0.02$	$11.62\pm 0.02$	$11.1\pm 0.2$	$9.0$
J0701+2401	$12.87\pm 0.02$	$12.27\pm 0.02$	$11.87\pm 0.02$	AAA	$11.67\pm 0.02$	$11.47\pm 0.02$	$11.3\pm 0.2$	$8.9$
J0703+0711	$12.78\pm 0.02$	$12.13\pm 0.03$	$11.76\pm 0.02$	AAA	$11.58\pm 0.02$	$11.36\pm 0.02$	$11.1\pm 0.2$	$8.7$
J0756+6234	$14.17\pm 0.03$	$13.55\pm 0.04$	$13.09\pm 0.04$	AAA	$12.89\pm 0.02$	$12.68\pm 0.02$	$12.2\pm 0.4$	$9.0$
J0817+5246	$14.33\pm 0.03$	$13.62\pm 0.04$	$13.21\pm 0.04$	AAA	$12.89\pm 0.02$	$12.62\pm 0.03$	$12.6\pm 0.5$	$8.7$
J0833+3706	$12.28\pm 0.03$	$11.63\pm 0.03$	$11.23\pm 0.02$	AAA	$10.98\pm 0.02$	$10.80\pm 0.02$	$10.5\pm 0.1$	$9.0$
J0844+7101	$14.50\pm 0.04$	$13.76\pm 0.04$	$13.44\pm 0.04$	AAA	$13.29\pm 0.02$	$13.09\pm 0.03$	$12.4\pm 0.5$	$9.0$
J1036+5932	$13.88\pm 0.03$	$13.30\pm 0.04$	$12.88\pm 0.03$	AAA	$12.62\pm 0.02$	$12.37\pm 0.02$	$11.6\pm 0.2$	$8.5$
J1133+1039	$13.42\pm 0.02$	$12.79\pm 0.03$	$12.41\pm 0.03$	AAA	$12.20\pm 0.02$	$11.98\pm 0.02$	$11.7\pm 0.3$	$8.5$
J1155+0922	$13.90\pm 0.03$	$13.30\pm 0.03$	$12.88\pm 0.03$	AAA	$12.74\pm 0.02$	$12.54\pm 0.03$	$11.8$	$8.9$
J1549+0151	$14.35\pm 0.03$	$13.67\pm 0.03$	$13.24\pm 0.03$	AAA	$13.10\pm 0.02$	$12.83\pm 0.03$	$12.1$	$8.8$
J1737+4705	$14.57\pm 0.03$	$13.84\pm 0.03$	$13.43\pm 0.04$	AAA	$13.12\pm 0.02$	$12.81\pm 0.02$	$12.0\pm 0.2$	$9.3$
J1938+6010	$14.48\pm 0.04$	$13.82\pm 0.04$	$13.40\pm 0.04$	AAA	$13.18\pm 0.02$	$12.97\pm 0.02$	$12.6\pm 0.3$	$9.3$
J2105+2514	$14.48\pm 0.03$	$13.77\pm 0.03$	$13.19\pm 0.04$	AAA	$12.97\pm 0.02$	$12.73\pm 0.03$	$12.0$	$9.1$
J2116+0341	$13.91\pm 0.03$	$13.25\pm 0.02$	$12.84\pm 0.03$	AAA	$12.54\pm 0.02$	$12.23\pm 0.02$	$11.7\pm 0.3$	$8.6$
J2126+2531	$12.61\pm 0.02$	$11.97\pm 0.02$	$11.57\pm 0.02$	AAA	$11.27\pm 0.02$	$11.01\pm 0.02$	$10.64\pm 0.08$	$8.8$
J2158+0409	$13.46\pm 0.03$	$12.86\pm 0.03$	$12.44\pm 0.03$	AAA	$12.23\pm 0.02$	$11.99\pm 0.02$	$11.8\pm 0.3$	$9.1$
J2158+5836	$13.06\pm 0.02$	$12.39\pm 0.03$	$11.93\pm 0.02$	AAA	$11.64\pm 0.02$	$11.41\pm 0.02$	$11.30\pm 0.11$	$9.7$
J2224-1852	$14.54\pm 0.04$	$13.91\pm 0.04$	$13.52\pm 0.05$	AAA	$13.28\pm 0.03$	$13.02\pm 0.03$	$12.6$	$9.0$
J2229-0444	$14.24\pm 0.03$	$13.62\pm 0.03$	$13.21\pm 0.03$	AAA	$13.03\pm 0.03$	$12.77\pm 0.03$	$11.9\pm 0.3$	$8.9$
J2251+2952	$13.71\pm 0.02$	$13.04\pm 0.03$	$12.67\pm 0.02$	AAA	$12.47\pm 0.02$	$12.29\pm 0.02$	$12.0\pm 0.3$	$8.7\pm 0.4$
J2310+3230	$13.97\pm 0.03$	$13.32\pm 0.03$	$12.91\pm 0.03$	AAA	$12.68\pm 0.03$	$12.48\pm 0.02$	$11.9\pm 0.2$	$8.7$
J2327+0450	$15.10\pm 0.03$	$14.36\pm 0.03$	$13.97\pm 0.05$	AAA	$13.50\pm 0.03$	$13.16\pm 0.03$	$12.0$	$8.6\pm 0.4$
J2333+3925	$14.33\pm 0.03$	$13.62\pm 0.04$	$13.19\pm 0.03$	AAA	$13.05\pm 0.02$	$12.79\pm 0.02$	$11.6$	$8.4$

). Distances were available from Gaia DR2 (Gaia Collaboration et al. [20]) and DR3 (Gaia Collaboration et al. [21]). Three different distance estimates (DR3 offers estimates based on parallax and photometry) were retrieved for the sample except for six targets for which there were only two estimates. All distances are in excellent agreement within the uncertainties (

Table A3. Gaia distance data. The first column contains the shortened 2MASS identification of the objects, followed by the corresponding Gaia ID in column two. Columns three to five show the recovered Gaia DR2 and DR3 distances. Column six gives weighted mean of the distances presented in columns three, four, and five.

2MASS ID	Gaia ID	${\mathit{d}}_{\mathbf{DR}2}$	${\mathit{d}}_{\mathbf{DR}3}$	${\mathit{d}}_{\mathbf{DR}3\mathbf{ph}}$	${\mathit{d}}_{\mathbf{WMDRs}}$
		[pc.]	[pc.]	[pc.]	[pc.]
J0005+0217	2739077223651402240	$39.0\pm 0.3$	$38.9\pm 0.3$	$39.0\pm 0.3$	$38.94\pm 0.17$
J0027-1813	2366831549112880000	$40.1\pm 0.3$	$39.7\pm 0.2$	−	$39.82\pm 0.17$
J0101+0336	2551477793805008256	$85.6\pm 5.5$	$88.6\pm 3.6$	$87.4\pm 3.9$	$87.6\pm 2.4$
J0113-1408	2456022519312921472	$35.7\pm 0.4$	$35.5\pm 0.2$	$35.4\pm 0.3$	$35.48\pm 0.16$
J0130-0211	2485433257060541184	$26.16\pm 0.13$	$26.02\pm 0.10$	−	$26.07\pm 0.08$
J0213-1343	5149965026866229760	$39.5\pm 0.6$	$39.3\pm 0.5$	$39.3\pm 0.6$	$39.4\pm 0.3$
J0311+0417	2781562554898432	$42.5\pm 0.6$	$42.6\pm 0.4$	$42.5\pm 0.4$	$42.5\pm 0.3$
J0318+0828	11064763467877504	−	$40.9\pm 0.3$	$40.9\pm 0.4$	$40.9\pm 0.2$
J0342+4629	247887099802005504	$34.2\pm 0.2$	$34.2\pm 0.2$	$34.3\pm 0.2$	$34.22\pm 0.12$
J0413+3709	177700805837431296	$19.35\pm 0.13$	$19.24\pm 0.08$	$19.25\pm 0.08$	$19.26\pm 0.05$
J0420+5624	276870604275149568	$32.40\pm 0.15$	$32.45\pm 0.12$	$32.43\pm 0.11$	$32.43\pm 0.07$
J0508+3319	181724125038647040	$18.9\pm 0.2$	$18.86\pm 0.19$	$18.85\pm 0.18$	$18.86\pm 0.11$
J0533+3740	190314025273114752	$26.72\pm 0.15$	$26.16\pm 0.06$	$26.16\pm 0.06$	$26.20\pm 0.04$
J0538-0808	3015405730029371392	$32.56\pm 0.11$	$32.40\pm 0.10$	$32.41\pm 0.10$	$32.45\pm 0.06$
J0614+3833	956200977271782144	$25.2\pm 0.2$	$25.25\pm 0.15$	$25.24\pm 0.15$	$25.23\pm 0.10$
J0701+2401	3380600854974394880	$35.7\pm 0.2$	$35.72\pm 0.16$	$35.65\pm 0.14$	$35.68\pm 0.10$
J0703+0711	3153877399801800576	$26.77\pm 0.16$	$26.58\pm 0.07$	$26.58\pm 0.09$	$26.60\pm 0.05$
J0756+6234	1085224509260180736	$46.7\pm 0.5$	$45.7\pm 0.3$	$45.7\pm 0.3$	$45.9\pm 0.2$
J0817+5246	1031554907167231872	$40.7\pm 0.6$	$40.9\pm 0.4$	$41.0\pm 0.6$	$40.9\pm 0.3$
J0833+3706	910087150363664000	$24.23\pm 0.09$	$24.01\pm 0.05$	$24.00\pm 0.04$	$24.02\pm 0.03$
J0844+7101	1121588966563321856	$67.6\pm 1.1$	$67.5\pm 0.8$	$67.8\pm 0.7$	$67.7\pm 0.5$
J1036+5932	1047702987768668928	$51.8\pm 0.4$	$51.9\pm 0.3$	$52.0\pm 0.4$	$51.9\pm 0.2$
J1133+1039	3916235366147580288	$35.3\pm 0.4$	$35.7\pm 0.2$	−	$35.62\pm 0.18$
J1155+0922	3912151844386051584	$55.4\pm 0.8$	$55.0\pm 0.5$	$54.9\pm 0.5$	$55.0\pm 0.3$
J1549+0151	4423184031270272896	$48.2\pm 0.7$	$48.0\pm 0.5$	$47.8\pm 0.5$	$48.0\pm 0.3$
J1737+4705	1363482108789712000	$37.0\pm 0.4$	$37.3\pm 0.3$	$37.2\pm 0.3$	$37.2\pm 0.2$
J1938+6010	2238983395052535936	$51.2\pm 0.5$	$51.6\pm 0.4$	$51.3\pm 0.4$	$51.4\pm 0.3$
J2105+2514	1841350340628210176	$101.9\pm 1.1$	$103.4\pm 0.8$	$103.5\pm 0.8$	$103.1\pm 0.5$
J2116+0341	1731827128389211520	$29.1\pm 0.3$	$29.0\pm 0.2$	$29.0\pm 0.2$	$29.01\pm 0.12$
J2126+2531	1798588409277193984	$18.11\pm 0.05$	$18.08\pm 0.03$	−	$18.09\pm 0.03$
J2158+0409	2695875380274977280	$32.2\pm 0.5$	$32.4\pm 0.3$	−	$32.4\pm 0.3$
J2158+5836	2199435611080216704	$24.01\pm 0.06$	$24.12\pm 0.05$	$24.11\pm 0.06$	$24.09\pm 0.03$
J2224-1852	6821768338025389440	$54.1\pm 0.9$	$53.1\pm 0.6$	$53.1\pm 0.6$	$53.3\pm 0.4$
J2229-0444	2626353401784545280	$51.1\pm 0.7$	$51.5\pm 0.5$	$51.4\pm 0.5$	$51.4\pm 0.3$
J2251+2952	1887751380587578112	$47.2\pm 0.4$	$47.3\pm 0.4$	$47.4\pm 0.3$	$47.3\pm 0.2$
J2310+3230	1910773333005191680	$45.7\pm 0.7$	$44.8\pm 0.4$	$44.7\pm 0.4$	$44.9\pm 0.3$
J2327+0450	2660317419045705344	$54.0\pm 2.3$	$57.4\pm 1.9$	$57.8\pm 2.1$	$56.6\pm 1.2$
J2333+3925	1920130348756557952	$50.4\pm 0.6$	$50.6\pm 0.5$	$50.4\pm 0.5$	$50.5\pm 0.3$

). Based on the recovered data, an individual distance value was calculated for each object as the weighted mean of the available estimates. These values are listed in the last columns of

Table 2. Spectral classification, atmospheric parameters, and distances for the stars in the sample. The spectral types are based on a combination of continuum and indices comparison with templates (original (SpT₁) and interpolated (SpT₂)) from the PyHammer2.0 libraries. Cited metallicities are also based on template metallicities from PyHammer2.0. T_eff and log g are obtained by comparison with BT-Settl (CIFIST) model spectra. Their errors reflect the spacing of the available models.

2MASS	SpT1	SpT2	[Fe/H]	Teff [K]	log g	d [pc]
ID	$\pm \mathbf{1.0}$	$\pm \mathbf{1.0}$	$\pm \mathbf{0.4}$	$\pm \mathbf{100}$	$\pm \mathbf{0.5}$
J0005+0217	M8.0	M8.0	−0.5	2600	5.0	$38.94\pm 0.17$
J0027-1813	M8.0	M8.0	+0.5	2600	5.0	$39.82\pm 0.17$
J0101+0336	M8.0	M8.0	+0.5	2600	5.0	$87.6\pm 2.4$
J0113-1408	M9.0	M9.0	+0.0	2500	5.5	$35.48\pm 0.16$
J0130-0211	M8.0	M8.0	+0.5	2700	5.5	$26.07\pm 0.08$
J0213-1343	M9.0	M9.5	+0.0	2600	5.5	$39.4\pm 0.3$
J0311+0417	M9.0	M9.0	+0.0	2600	5.5	$42.5\pm 0.3$
J0318+0828	M8.0	M8.0	+0.5	2600	5.0	$40.9\pm 0.2$
J0342+4629	M9.0	M9.0	+0.0	2600	5.0	$34.22\pm 0.12$
J0413+3709	L1.0	L1.0	+0.0	1850	5.0	$19.26\pm 0.05$
J0420+5624	M8.0	M7.5	+0.5	2700	5.0	$32.43\pm 0.07$
J0508+3319	L2.0	L1.5	+0.0	1800	5.0	$18.86\pm 0.11$
J0533+3740	M8.0	M7.5	+0.0	2800	5.5	$26.20\pm 0.04$
J0538-0808	M8.0	M8.5	+0.5	2800	5.5	$32.45\pm 0.06$
J0614+3833	L0.0	M9.5	+0.0	2600	5.5	$25.23\pm 0.10$
J0701+2401	M8.0	M7.5	+0.0	2800	5.0	$35.68\pm 0.10$
J0703+0711	M8.0	M8.0	−0.5	2600	5.0	$26.60\pm 0.05$
J0756+6234	M8.0	M8.0	+0.5	2600	5.0	$45.9\pm 0.2$
J0703+0711	M8.0	M8.0	−0.5	2600	5.0	$26.60\pm 0.05$
J0756+6234	M8.0	M8.0	+0.5	2600	5.0	$45.9\pm 0.2$
J0817+5246	M9.0	M9.0	+0.0	2600	5.0	$40.9\pm 0.3$
J0833+3706	M8.0	M7.5	+0.0	2700	5.5	$24.02\pm 0.03$
J0844+7101	M8.0	M8.5	+0.5	2800	5.0	$67.7\pm 0.5$
J1036+5932	M8.0	M8.5	+0.5	2700	5.0	$51.9\pm 0.2$
J1133+1039	M8.0	M8.0	+0.5	2600	5.0	$35.62\pm 0.18$
J1155+0922	M8.0	M8.0	−0.5	2800	5.0	$55.0\pm 0.3$
J1549+0151	M8.0	M8.0	+0.5	2600	5.0	$48.0\pm 0.3$
J1737+4705	L0.0	M9.5	+0.0	2500	5.5	$37.2\pm 0.2$
J1938+6010	M8.0	M8.0	+0.5	2700	5.0	$51.4\pm 0.3$
J2105+2514 1	dCM	M0.0	-0.5	4100	5.5	$103.1\pm 0.5$
J2116+0341	M8.0	M8.0	+0.5	2500	5.0	$29.01\pm 0.12$
J2126+2531	M9.0	M9.0	+0.0	2600	5.5	$18.09\pm 0.03$
J2158+0409	M8.0	M8.0	+0.5	2600	5.5	$32.4\pm 0.3$
J2158+5836	M8.0	M9.0	+0.5	2600	5.5	$24.09\pm 0.03$
J2224-1852	M8.0	M9.0	+0.5	2600	5.0	$53.3\pm 0.4$
J2229-0444	M8.0	M8.0	+0.5	2700	5.0	$51.4\pm 0.3$
J2251+2952	M8.0	M7.5	+0.5	2800	5.5	$47.3\pm 0.2$
J2310+3230	M8.0	M9.0	+0.5	2600	5.0	$44.9\pm 0.3$
J2327+0450	M9.0	M9.0	+0.0	2500	5.0	$56.6\pm 1.2$
J2333+3925	M8.0	M9.0	+0.5	2600	5.0	$50.5\pm 0.3$

¹ This is a dwarf carbon star.

and Table A3 and were utilised for the calculation of the Gaia and 2MASS absolute magnitudes presented in

Table A4. 2MASS NIR and Gaia optical absolute magnitudes. The first column contains the shortened 2MASS identification of the objects. Columns two, three, and four represent the calculated absolute magnitudes in the 2MASS J, H, and Ks filters, respectively. Column five shows the (J-Ks) colour. Columns six to eight contain the absolute G, BP, and RP magnitudes of the objects, followed by the (BP-RP) colour in column nine.

2MASS ID	${\mathit{M}}_{\mathit{J}}$	${\mathit{M}}_{\mathit{H}}$	${\mathit{M}}_{\mathbf{Ks}}$	($\mathit{J}-\mathbf{Ks}$)	${\mathit{M}}_{\mathit{G}}$	${\mathit{M}}_{\mathbf{BP}}$	${\mathit{M}}_{\mathbf{RP}}$	(BP-RP)
	[mag.]	[mag.]	[mag.]	[mag.]	[mag.]	[mag.]	[mag.]	[mag.]
J0005+0217	$10.73\pm 0.03$	$10.10\pm 0.04$	$9.72\pm 0.03$	$1.01\pm 0.02$	$14.884\pm 0.010$	$17.82\pm 0.13$	$13.33\pm 0.01$	$4.49\pm 0.13$
J0027-1813	$10.45\pm 0.02$	$9.85\pm 0.03$	$9.43\pm 0.03$	$1.02\pm 0.03$	$14.571\pm 0.010$	$17.60\pm 0.09$	$13.05\pm 0.01$	$4.55\pm 0.09$
J0101+0336	$10.71\pm 0.08$	$9.94\pm 0.09$	$9.59\pm 0.09$	$1.12\pm 0.03$	$14.77\pm 0.06$	$16.55\pm 0.18$	$13.24\pm 0.06$	$3.31\pm 0.17$
J0113-1408	$11.45\pm 0.03$	$10.74\pm 0.03$	$10.37\pm 0.03$	$1.08\pm 0.03$	$16.013\pm 0.011$	$18.57\pm 0.16$	$14.42\pm 0.01$	$4.15\pm 0.16$
J0130-0211	$10.95\pm 0.02$	$10.29\pm 0.03$	$9.94\pm 0.03$	$1.01\pm 0.03$	$15.219\pm 0.007$	$18.43\pm 0.07$	$13.66\pm 0.01$	$4.77\pm 0.07$
J0213-1343	$11.51\pm 0.04$	$10.82\pm 0.04$	$10.36\pm 0.04$	$1.14\pm 0.04$	$16.139\pm 0.017$	$18.26\pm 0.15$	$14.57\pm 0.02$	$3.69\pm 0.15$
J0311+0417	$11.09\pm 0.03$	$10.43\pm 0.04$	$10.02\pm 0.03$	$1.07\pm 0.03$	$15.416\pm 0.016$	$18.3\pm 0.2$	$13.84\pm 0.02$	$4.5\pm 0.2$
J0318+0828	$10.73\pm 0.03$	$10.03\pm 0.03$	$9.63\pm 0.03$	$1.10\pm 0.02$	$14.980\pm 0.011$	$17.79\pm 0.14$	$13.43\pm 0.01$	$4.36\pm 0.14$
J0342+4629	$11.12\pm 0.03$	$10.43\pm 0.03$	$10.01\pm 0.03$	$1.11\pm 0.03$	$15.438\pm 0.008$	$18.36\pm 0.11$	$13.87\pm 0.01$	$4.49\pm 0.11$
J0413+3709	$12.37\pm 0.03$	$11.60\pm 0.04$	$11.08\pm 0.03$	$1.29\pm 0.02$	$17.197\pm 0.006$	$19.67\pm 0.16$	$15.59\pm 0.01$	$4.08\pm 0.16$
J0420+5624	$10.41\pm 0.02$	$9.76\pm 0.02$	$9.38\pm 0.02$	$1.03\pm 0.02$	$14.425\pm 0.006$	$17.45\pm 0.05$	$12.92\pm 0.01$	$4.52\pm 0.05$
J0508+3319	$12.84\pm 0.03$	$11.86\pm 0.04$	$11.25\pm 0.03$	$1.59\pm 0.03$	$17.763\pm 0.013$	$20.3\pm 0.2$	$16.22\pm 0.02$	$4.1\pm 0.2$
J0533+3740	$10.45\pm 0.02$	$9.81\pm 0.02$	$9.43\pm 0.02$	$1.02\pm 0.02$	$14.391\pm 0.005$	$17.30\pm 0.04$	$12.92\pm 0.01$	$4.38\pm 0.04$
J0538-0808	$10.52\pm 0.03$	$9.83\pm 0.03$	$9.45\pm 0.03$	$1.07\pm 0.03$	$14.505\pm 0.005$	$17.56\pm 0.07$	$12.99\pm 0.01$	$4.57\pm 0.07$
J0614+3833	$11.51\pm 0.03$	$10.74\pm 0.03$	$10.24\pm 0.02$	$1.27\pm 0.02$	$16.133\pm 0.010$	$18.84\pm 0.12$	$14.55\pm 0.01$	$4.30\pm 0.12$
J0701+2401	$10.11\pm 0.02$	$9.51\pm 0.03$	$9.11\pm 0.02$	$1.00\pm 0.02$	$13.980\pm 0.007$	$16.73\pm 0.05$	$12.50\pm 0.01$	$4.23\pm 0.05$
J0703+0711	$10.66\pm 0.02$	$10.01\pm 0.03$	$9.64\pm 0.02$	$1.02\pm 0.02$	$14.745\pm 0.005$	$17.86\pm 0.07$	$13.24\pm 0.01$	$4.62\pm 0.07$
J0756+6234	$10.86\pm 0.03$	$10.24\pm 0.04$	$9.78\pm 0.04$	$1.08\pm 0.04$	$15.030\pm 0.010$	$17.91\pm 0.10$	$13.48\pm 0.01$	$4.4\pm 0.1$
J0817+5246	$11.27\pm 0.03$	$10.56\pm 0.04$	$10.15\pm 0.04$	$1.12\pm 0.04$	$15.820\pm 0.016$	$18.3\pm 0.2$	$14.24\pm 0.02$	$4.09\pm 0.22$
J0833+3706	$10.38\pm 0.03$	$9.73\pm 0.03$	$9.32\pm 0.02$	$1.05\pm 0.02$	$14.535\pm 0.004$	$17.36\pm 0.05$	$13.05\pm 0.01$	$4.31\pm 0.05$
J0844+7101	$10.34\pm 0.04$	$9.61\pm 0.04$	$9.28\pm 0.04$	$1.06\pm 0.04$	$14.269\pm 0.016$	$17.02\pm 0.13$	$12.81\pm 0.02$	$4.21\pm 0.13$
J1036+5932	$10.31\pm 0.03$	$9.72\pm 0.04$	$9.30\pm 0.03$	$1.00\pm 0.03$	$14.274\pm 0.009$	$17.00\pm 0.10$	$12.78\pm 0.01$	$4.2\pm 0.1$
J1133+1039	$10.67\pm 0.03$	$10.03\pm 0.03$	$9.65\pm 0.03$	$1.01\pm 0.03$	$14.905\pm 0.011$	$18.21\pm 0.15$	$13.37\pm 0.01$	$4.85\pm 0.15$
J1155+0922	$10.19\pm 0.03$	$9.59\pm 0.03$	$9.17\pm 0.04$	$1.02\pm 0.03$	$13.946\pm 0.012$	$16.57\pm 0.05$	$12.48\pm 0.01$	$4.09\pm 0.05$
J1549+0151	$10.94\pm 0.04$	$10.27\pm 0.03$	$9.84\pm 0.04$	$1.10\pm 0.03$	$15.083\pm 0.014$	$17.53\pm 0.12$	$13.52\pm 0.02$	$4.02\pm 0.12$
J1737+4705	$11.72\pm 0.03$	$10.98\pm 0.04$	$10.58\pm 0.04$	$1.14\pm 0.04$	$16.384\pm 0.012$	$18.19\pm 0.15$	$14.82\pm 0.02$	$3.36\pm 0.15$
J1938+6010	$10.92\pm 0.04$	$10.27\pm 0.05$	$9.85\pm 0.04$	$1.08\pm 0.04$	$15.127\pm 0.013$	$17.3\pm 0.2$	$13.58\pm 0.02$	$3.7\pm 0.2$
J2105+2514	$9.42\pm 0.04$	$8.70\pm 0.03$	$8.13\pm 0.04$	$1.29\pm 0.04$	$11.631\pm 0.011$	$12.591\pm 0.013$	$10.68\pm 0.01$	$1.91\pm 0.01$
J2116+0341	$11.59\pm 0.03$	$10.94\pm 0.02$	$10.53\pm 0.03$	$1.07\pm 0.03$	$16.093\pm 0.010$	$18.78\pm 0.15$	$14.50\pm 0.01$	$4.28\pm 0.15$
J2126+2531	$11.33\pm 0.02$	$10.69\pm 0.02$	$10.29\pm 0.02$	$1.04\pm 0.02$	$15.715\pm 0.005$	$19.27\pm 0.09$	$14.16\pm 0.01$	$5.11\pm 0.09$
J2158+0409	$10.91\pm 0.03$	$10.30\pm 0.04$	$9.88\pm 0.03$	$1.03\pm 0.03$	$15.25\pm 0.02$	$18.21\pm 0.18$	$13.70\pm 0.02$	$4.51\pm 0.18$
J2158+5836	$11.15\pm 0.02$	$10.48\pm 0.03$	$10.03\pm 0.02$	$1.13\pm 0.02$	$15.561\pm 0.004$	$18.89\pm 0.07$	$13.98\pm 0.01$	$4.91\pm 0.07$
J2224-1852	$10.91\pm 0.04$	$10.28\pm 0.04$	$9.89\pm 0.05$	$1.02\pm 0.05$	$15.163\pm 0.017$	$17.8\pm 0.2$	$13.58\pm 0.02$	$4.20\pm 0.24$
J2229-0444	$10.68\pm 0.03$	$10.07\pm 0.03$	$9.66\pm 0.03$	$1.02\pm 0.03$	$14.846\pm 0.013$	$17.5\pm 0.3$	$13.31\pm 0.02$	$4.1\pm 0.3$
J2251+2952	$10.34\pm 0.03$	$9.67\pm 0.03$	$9.30\pm 0.03$	$1.05\pm 0.02$	$14.269\pm 0.010$	$17.03\pm 0.07$	$12.78\pm 0.01$	$4.24\pm 0.07$
J2310+3230	$10.71\pm 0.03$	$10.06\pm 0.04$	$9.65\pm 0.03$	$1.06\pm 0.03$	$15.034\pm 0.015$	$17.82\pm 0.10$	$13.53\pm 0.02$	$4.3\pm 0.1$
J2327+0450	$11.33\pm 0.06$	$10.59\pm 0.05$	$10.21\pm 0.06$	$1.12\pm 0.05$	$15.72\pm 0.05$	$17.2\pm 0.2$	$14.22\pm 0.05$	$2.94\pm 0.23$
J2333+3925	$10.82\pm 0.03$	$10.11\pm 0.04$	$9.67\pm 0.03$	$1.14\pm 0.03$	$15.145\pm 0.013$	$17.69\pm 0.19$	$13.58\pm 0.02$	$4.11\pm 0.19$

, as well as the bolometric and H$\alpha$ luminosities.

Combining the GTC spectra and the archival data, we obtained spectral classification, estimates of effective temperatures and log g, bolometric magnitudes and luminosities, as well as estimates of the magnetic activity of all our targets. These are presented in the following sections, where one is dedicated to a particular object—the dwarf carbon star 2MASS J21051653+2514486 (LSR J2105+2514).

3. Spectral Classification and Atmospheric Parameters

3.1. Spectral Classification

For the spectral classification, we used the M and L single dwarf star templates and indices definitions used in PyHammer2.0 [35,36]. The templates are empirical stellar spectra with resolution∼R2000, created by combining multiple SDSS spectra for given metallicity and log g bins. However, the metallicity bins expand only to M8 spectral type; thus, for all later types, solar metallicity is assumed. To these, Roulston et al. [35] added templates for three subtypes of dwarf carbon stars (dCG, dCK, and dCM) depending on the spectral type of the cool dwarf. From the indices, we used all that are within the wavelength range of our observations (5200–10,000 Å). In addition, because this library does not provide the 0.5 subtype and because the spectral type–T_eff relation for M6 to L2 dwarfs is fairly linear [37], we decided to perform linear interpolation between the nearest neighbours (e.g., M7 and M8) to obtain the missing intermediate spectral 0.5 subtypes in the range M7 to L2. The interpolation is performed on the flux arrays only.

${\chi }^2$ was then calculated between observed and template flux (normalised at 7950–8000 Å) in a fitting region 6000–8800 Å for each object. An index score was also obtained as the squared difference in all spectral indices. The total score for each spectrum is the sum of the ${\chi }^2$ and the index score, the latter multiplied by a factor of 10 to increase its weight. The best match is the template with the lowest total score.

We performed these steps using both the original and interpolated subtypes The results are shown in column 2 (SpT₁, using the original templates) and column 3 (SpT₂, using the interpolated ones) in Table 2. To estimate the uncertainties, we used the weighted means of the six closest templates in the grid, which gave us as a result ± 1.0 spectral subtype. Because the templates for spectral subtypes M7 and M8 also have metallicity included, we list this metallicity in column 4, and the error (0.4 dex) is the one given by Kesseli et al. [36]. For the later types (M8.5 to L2), solar metallicity is assumed. Figure 2 shows the best matches of the observed spectra to the interpolated 0.5 subclass templates for the objects, where there is a difference in the determined spectral type. The best fits using only the original templates for the whole sample are shown in Figure A1 and Figure A2.

As shown in Table 2 and Figure 2, Figure A1, and Figure A2, both approaches—using the original spectral-type templates and the interpolated subtypes—yield the same results for the majority of the objects. For the remaining dwarfs, the spectral types derived are very similar and agree within the uncertainties of the fit. Visual inspection of the plots indicates that in most cases, the interpolated spectra provide a slightly better match, with possible exceptions for 2MASS J2158+5836 and 2MASS J2333+3925, likely due to the higher noise level in their observed spectra.

As an independent experiment, a subsample of our objects was analysed by a team of summer school students utilising standard IRAF [38] reduction routines and PyHammer [36] (see Cabello et al. [39]). Within the uncertainties estimated earlier (±1.0 spectral subtype), there is an excellent agreement between both analyses, and an exact match of the spectral classification is observed for the majority of the objects.

Comparison with results from the literature, obtained by different methods, also gives excellent matches in almost all cases. Ahmed and Warren [40] report on a sample of 33665 M7–M9.5 dwarfs, for which they calculated spectral type based on photometry from SDSS and the UKIRT Infrared Deep Sky Survey. They give an error of their classification of ≈0.5 subtype. We find six of our targets in their sample. For 2MASS J0005+0217 (M8), 2MASS J0101+0336 (M8), 2MASS J0311+0417 (M9), and 2MASS J1549+0151 (M8) they give exactly the same type, while for 2MASS J2327+0450 (M9) they give spectral type M9.5 and for 2MASS J1155+0922 (M8) their classification is M7. In another study, Cheng et al. [41] used near-infrared spectra to classify 51 nearby ultracool dwarfs. We find two that are also in our sample. For 2MASS J0508+3319 (L2 or L1.5) they give L2 based on IR spectroscopy and L1 as photometric spectral type. This again is an excellent match. For the other dwarf, 2MASS J1737+4705 (L0 or M9.5), however, they give an earlier type of M8 or M8.5, where the second value is the photometric spectral type. Even though in this case the difference is of ∼1–2 spectral subtypes, it is still a very good match.

3.2. Atmospheric Parameters

To estimate the effective temperature (T_eff) and surface gravity (log g), we used the BT-Settl (CIFIST) model spectra [22,23,24], downloaded from the SVO server [42] (https://svo2.cab.inta-csic.es/theory/newov2/index.php, accessed on 17 July 2025) for the temperature ranges 4500 K ≤ T_eff ≤ 1300 K and 3.5 ≤ log g ≤ 5.5. These are theoretical stellar atmosphere models aimed at low-mass stars, brown dwarfs, and exoplanets. They treat molecular and dust opacities (TiO, VO, H₂O, CO, CH₄, FeH, CrH, etc.), including condensation of dust grains.

The models were convolved using a Gaussian kernel to the target resolution of R1000 and interpolated onto a common grid with the observed spectra. The grid was chosen to be within the 5200–10,000 Å limits, i.e., where we have observational data. Additionally, the models were smoothed using a Savitzky–Golay filter using a small window length (31) to ensure preserving the spectral features.

For each observed spectrum, ${\chi }^2$ minimisation across the model grid was performed, and weighted ${\chi }^2$ was calculated. T_eff and log g were estimated from the best-fit model, and their uncertainties are based on $\mathsf{\Delta }{\chi }^2$ and the model grid spacing. The results are shown in Table 2. In Figure 3, we show a sample of fits for different temperatures and log g for dwarfs of different spectral types.

There are only two objects we can directly compare to other recent studies in the literature. Cheng et al. [41] give T_eff = 2000 ± 60 K for 2MASS J0508+3319, i.e., ∼150 K hotter than our estimate, which we find a reasonable match. For 2MASS J1737+4705, however, these authors give T_eff = 1200 ± 100 K, which we find to be too low for a dwarf of late-M spectral type. Lacking direct comparisons, we looked at the general trend of the effective temperature with spectral type. Rajpurohit et al. [43], based on mid- and low-resolution optical spectra, estimate effective temperatures of ∼2600–2700 K for their stars of spectral types M7.5 to M8 and ∼2500 K for the M8.5–M9 sample. Filippazzo et al. [37], using a combination of optical, near-infrared, and mid-infrared spectra and photometry of field ultracool dwarfs, derive a scale of effective temperature with spectral type, which they fit with a sixth-order polynomial. Assuming all our dwarfs are field objects, we find that our estimates for most of the temperatures of the M7.5–M9 dwarfs are ∼200 K higher and the T_eff for the L dwarfs are ∼200 K lower than predicted by the fit. However, the general trend is preserved, and for all objects we are within the 2–3$\sigma$ region of the results from these studies.

The estimated surface gravities are consistent with old late-M and early-L dwarfs from the field [37]. This is supported also by the results reported by Cheng et al. [41] for 2MASS J0508+3319 and 2MASS J1737+4705.

4. Chromospheric Activity

One of the most widely used indicators of chromospheric magnetic activity is H$\alpha$ emission, especially in ultracool dwarfs, where the flux is mostly concentrated towards the redder wavelengths. Although initially expected to drop significantly in the ultracool dwarf regime, it was found that almost all late-M dwarfs exhibit H$\alpha$ emission [44,45], with fractions reaching up to ∼90% by L0 and dropping by ∼50% by L5 [17].

To search for chromospheric activity, we first defined the underlying stellar continuum using regions on either side of the H$\alpha$ line position, which we fit with a second-order polynomial. The resultant continuum was then subtracted from the data in a broad wavelength window around H$\alpha$ (6542–6588 Å). The line was fit with a four-parameter Gaussian model (amplitude, centre, width, and offset) to derive the flux (F_Hα), FWHM, and equivalent width (EW_Hα). As a result, 28 of the ultracool dwarfs and the dC star were found to be emitters (

Table 3. H$\alpha$ equivalent widths (EW_Hα), fluxes (F_Hα), and luminosities (L_Hα), as well as bolometric magnitudes (M_bol) and luminosities (L_bol) and the ratio of H$\alpha$ to bolometric luminosity (log(L_Hα/L_bol)) for the sample.

2MASS ID	EWHα	FHα$\times {10}^{16}$	LHα$\times {10}^{25}$	Mbol	Lbol$\times {10}^{30}$	log(LHα/Lbol)
Short	[Å]	[ $\mathrm{erg}{\mathrm{s}}^{-1}{\mathrm{cm}}^{-2}$ ]	[ $\mathrm{erg}{\mathrm{s}}^{-1}$ ]	[mag]	[ $\mathrm{erg}{\mathrm{s}}^{-1}$ ]
J0005+0217	$6.654\pm 1.714$	$4.159\pm 0.135$	$7.546\pm 0.254$	$12.69\pm 0.06$	$2.529\pm 0.139$	$-4.525\pm 0.028$
J0027-1813	$1.294\pm 0.771$	$2.511\pm 0.141$	$4.764\pm 0.270$	$12.41\pm 0.05$	$3.273\pm 0.150$	$-4.837\pm 0.032$
J0101+0336	$4.624\pm 1.421$	$0.593\pm 0.029$	$5.446\pm 0.402$	$12.67\pm 0.09$	$2.576\pm 0.213$	$-4.676\pm 0.048$
J0101+0336	$8.588\pm 1.758$	$0.897\pm 0.024$	$8.235\pm 0.501$			$-4.495\pm 0.044$
J0113-1408	$62.702\pm 1.328$	$13.330\pm 0.110$	$20.077\pm 0.024$	$13.4\pm 0.04$	$1.315\pm 0.048$	$-3.816\pm 0.017$
J0130-0211	$14.958\pm 1.696$	$14.890\pm 0.090$	$12.108\pm 0.106$	$12.91\pm 0.05$	$2.065\pm 0.095$	$-4.232\pm 0.020$
J0213-1343		$0.000\pm 0.009$	<$0.051$	$13.46\pm 0.05$	$1.244\pm 0.057$	<$-6.392$
J0311+0417	$8.769\pm 1.519$	$2.456\pm 0.070$	$5.308\pm 0.169$	$13.04\pm 0.04$	$1.832\pm 0.067$	$-4.538\pm 0.021$
J0318+0828	$10.251\pm 1.444$	$5.833\pm 0.089$	$11.675\pm 0.212$	$12.69\pm 0.06$	$2.529\pm 0.139$	$-4.336\pm 0.025$
J0342+4629	$7.028\pm 1.574$	$3.178\pm 0.108$	$4.453\pm 0.154$	$13.07\pm 0.04$	$1.782\pm 0.065$	$-4.602\pm 0.022$
J0413+3709		$0.000\pm 0.050$	<$0.067$	$14.28\pm 0.04$	$5.847\pm 0.021$	<$-5.941$
J0420+5624	$1.453\pm 1.570$	$2.224\pm 0.222$	$2.799\pm 0.280$	$12.37\pm 0.05$	$3.396\pm 0.156$	$-5.084\pm 0.048$
J0508+3319		$0.000\pm 0.092$	<$0.117$	$14.67\pm 0.09$	$4.082\pm 0.033$	<$-5.541$
J0533+3740	$36.632\pm 1.135$	$99.810\pm 0.728$	$81.976\pm 0.648$	$12.41\pm 0.05$	$3.273\pm 0.150$	$-3.601\pm 0.020$
J0538-0808	$1.795\pm 1.038$	$4.268\pm 0.231$	$5.377\pm 0.291$	$12.48\pm 0.06$	$3.068\pm 0.169$	$-4.756\pm 0.033$
J0614+3833	$6.247\pm 0.938$	$2.392\pm 0.095$	$1.822\pm 0.073$	$13.45\pm 0.07$	$1.255\pm 0.080$	$-4.838\pm 0.033$
J0701+2401	$3.156\pm 1.080$	$10.510\pm 0.404$	$16.009\pm 0.623$	$12.07\pm 0.05$	$4.476\pm 0.206$	$-4.447\pm 0.026$
J0703+0711	$12.152\pm 1.645$	$21.360\pm 0.412$	$18.083\pm 0.355$	$12.62\pm 0.05$	$2.697\pm 0.124$	$-4.174\pm 0.022$
J0756+6234	$7.359\pm 1.604$	$3.001\pm 0.053$	$7.565\pm 0.149$	$12.82\pm 0.06$	$2.243\pm 0.123$	$-4.472\pm 0.025$
J0817+5246	$11.271\pm 1.183$	$2.639\pm 0.067$	$5.282\pm 0.155$	$13.22\pm 0.04$	$1.552\pm 0.057$	$-4.468\pm 0.020$
J0833+3706	$21.754\pm 1.170$	$58.79\pm 0.841$	$40.585\pm 0.590$	$12.34\pm 0.06$	$3.491\pm 0.192$	$-3.935\pm 0.025$
J0844+7101		$0.000\pm 0.025$	<$0.418$	$12.3\pm 0.06$	$3.622\pm 0.200$	<$-5.938$
J1036+5932		$0.000\pm 0.029$	<$0.278$	$12.27\pm 0.06$	$3.723\pm 0.205$	<$-6.127$
J1133+1039	$26.834\pm 1.393$	$21.010\pm 0.255$	$31.895\pm 0.504$	$12.63\pm 0.06$	$2.672\pm 0.147$	$-3.923\pm 0.025$
J1155+0922	$1.818\pm 1.857$	$1.428\pm 0.163$	$5.168\pm 0.593$	$12.15\pm 0.06$	$4.158\pm 0.229$	$-4.906\pm 0.055$
J1549+0151	$9.831\pm 1.622$	$2.930\pm 0.093$	$8.077\pm 0.275$	$12.9\pm 0.06$	$2.084\pm 0.115$	$-4.412\pm 0.028$
J1737+4705	$5.670\pm 0.746$	$5.398\pm 0.032$	$0.894\pm 0.054$	$13.66\pm 0.07$	$1.035\pm 0.066$	$-5.064\pm 0.038$
J1938+6010	$3.616\pm 1.093$	$1.418\pm 0.055$	$4.482\pm 0.182$	$12.88\pm 0.06$	$2.123\pm 0.173$	$-4.675\pm 0.030$
J2105+2514 1
J2116+0341		$0.000\pm 0.005$	<$0.157$	$13.55\pm 0.06$	$1.145\pm 0.063$	<$-5.972$
J2126+2531	$2.099\pm 1.310$	$2.348\pm 0.127$	$0.919\pm 0.050$	$13.28\pm 0.04$	$1.468\pm 0.054$	$-5.367\pm 0.034$
J2158+0409	$7.892\pm 1.437$	$4.918\pm 0.213$	$6.177\pm 0.291$	$12.87\pm 0.06$	$2.142\pm 0.118$	$-4.444\pm 0.029$
J2158+5836		$0.000\pm 0.237$	<$0.049$	$13.11\pm 0.05$	$1.717\pm 0.079$	<$-6.638$
J2224-1852	$8.144\pm 1.530$	$1.619\pm 0.091$	$5.503\pm 0.320$	$12.87\pm 0.06$	$2.142\pm 0.118$	$-4.682\pm 0.035$
J2229-0444	$7.029\pm 1.385$	$2.710\pm 0.084$	$8.567\pm 0.284$	$12.64\pm 0.06$	$2.648\pm 0.146$	$-4.626\pm 0.028$
J2251+2952	$1.581\pm 1.195$	$2.060\pm 0.166$	$5.514\pm 0.447$	$12.3\pm 0.06$	$3.622\pm 0.200$	$-4.670\pm 0.042$
J2310+3230	$9.078\pm 1.052$	$3.400\pm 0.086$	$8.201\pm 0.234$	$12.67\pm 0.06$	$2.576\pm 0.142$	$-4.253\pm 0.031$
J2327+0450		$0.000\pm 0.005$	<$0.061$	$13.28\pm 0.07$	$1.468\pm 0.094$	<$-6.582$
J2333+3925		$0.000\pm 0.005$	<$0.050$	$12.78\pm 0.06$	$2.327\pm 0.128$	<$-6.420$

¹ See Table 4.

).

Using the M_J and M_K absolute magnitudes and the bolometric corrections (BC) from [46], we calculated the respective bolometric magnitudes. The comparison showed an excellent match. We used these bolometric magnitudes (based on M_J) to calculate the bolometric luminosities. Then we calculated the ratios of H$\alpha$ to bolometric luminosities. For the stars with no activity indicators, we estimated the 3$\sigma$ upper limits of the H$\alpha$ luminosity and log($L_{\mathrm{H}\alpha }/L_{\mathrm{bol}}$). These results are shown in Table 3. To our knowledge, this is the first time chromospheric activity has been reported from any of these stars.

Because we only have one epoch of observation, we cannot determine the level of variability of the emission. Similarly, for the objects with upper limits, we cannot definitely say whether they are inactive in general or only at the time of observation. Exceptions are 2MASS J0101+0336, which displayed a rise in the emission in the second half of our ≈1.5 h observations (Table 3), and the dwarf carbon star LSR J2105+2514, for which we obtained additional observations (

Table 4. Equivalent width (EW), full width at half maximum (FWHM), and measured flux (F) of the H$\alpha$ and H$\beta$ emission lines in the spectrum of LSR J2105+2514 in three epochs. The last column gives the respective luminosities (${\mathrm{L}}_{{\mathrm{H}}_i}$) of the two lines.

Date	Line	EW	FWHM	Flux $\times {10}^{-16}$	${\mathrm{L}}_{{\mathbf{H}}_{\mathit{i}}}\times {10}^{26}$
[yyyy mm dd]		[Å]	[Å]	[erg s−1 cm−2]	[erg s−1]
2016 10 10	H$\alpha$	$0.378\pm 0.043$	$6.529\pm 0.223$	$3.079\pm 0.136$	$3.917\pm 0.177$
2016 12 06	H$\alpha$	$0.819\pm 0.050$	$8.097\pm 0.168$	$3.689\pm 0.099$	$4.693\pm 0.134$
2016 12 07	H$\alpha$	$0.987\pm 0.039$	$9.012\pm 0.155$	$116.357\pm 2.572$	$148.017\pm 3.572$
2016 12 06	H$\beta$	$1.417\pm 5.851$	$6.799\pm 0.170$	$1.806\pm 0.0596$	$2.297\pm 0.079$

). Further monitoring observations are needed to better characterise the H$\alpha$ emission and its variability.

5. 2MASS J21051653+2514486 (LSR J2105+2514)—A Dwarf Carbon Star

Dwarf carbon (dC) stars are main sequence stars with spectra showing dominant absorption bands of C₂, CH, or CN (Figure 4a). Since dwarf stars are too cool to produce carbon, the suggestion is that they have undergone mass transfer of carbon-rich matter from the wind of a more massive companion, evolving on the asymptotic giant branch at the time and later evolving to a white dwarf [47]. Due to the different C/O ratio, dC stars may display a shift in the optical and near-infrared colours in comparison with the normal O-rich dwarfs. The typical absorption bands for the dC stars are a result of the C/O > 1 ratio, causing depletion of oxygen (locked in carbon compounds), which impedes the formation of TiO and VO molecules, which are basic features of M dwarfs [47]. One of the characteristics which distinguishes dCs from other carbon stars is their high proper motion (>11 mas yr⁻¹, [48]). The distances of the known dCs range from ∼58 pc [49] to ∼200–400 pc [50].

LSR J2105+2514 was first classified as a dwarf carbon star by Lowrance et al. [50] based on optical spectroscopy. They do not report indications of chromospheric activity and estimate the upper limit on distance (within 200 pc) based on the assumption that the system belongs to our galaxy gravitationally.

We misclassified LSR J2105+2514 as a potential late-type M dwarf based on its infrared colours and high proper motion. After the data reduction of the R1000R observation, it was clear that it is a dC star, but because these objects are quite rare and there is not much information about them in the literature, we requested and obtained additional R1000B and R1000R observations. A spectrum combining the blue and red parts (both from 6 December 2016) is shown in Figure 4a.

For the spectral classification, we used the templates from Roulston et al. [35], created by co-adding individual spectra from identified dC stars from SDSS. They are for three subtypes, dCG, dCK, and dCM, where the dwarf star is, respectively, of G, K, or M spectral class. The ${\chi }^2$ fit to the combined spectrum of the dwarfs indicates that its classification is dCM (Figure 4b). Compared to the “normal” M templates, the best fit is M0.0 $\pm 1.0$ with metallicity [Fe/H] = −0.5. As for the atmospheric parameters, we get as the best fit T_eff $=4000.000\pm 1400.000$ K and logg = 5.5 ± 0.5 (Figure 4c). The huge error margin in temperature is not very surprising considering the “best” fit is understandably poor—missing are key molecular bands and there are carbon features instead. Using only the red part yields the following best-fit parameters: T_eff $=4100.000\pm 100.000$ K and logg = 5.5 ± 0.5 (Figure 4d). Such temperature is consistent with the estimate of the effective temperature of the first discovered dC, G77-61 [49].

In a study of dCs in the SDSS spectroscopy database, Green [48] found that only about 3% of them show Balmer emission lines. Until now, LSR J2105+2514 was not among them. However, our observations revealed for the first time H$\alpha$ and H$\beta$ emission from this object. The calculated fluxes for H$\alpha$ were different for the three observations, which suggests variable activity. H$\alpha$ was detected in three epochs, while H$\beta$ was detected only once because there was no other observation with the blue grism. Close-ups of the fits to the two lines are shown in Figure 5, and the properties of the lines are listed in Table 4. In normal red dwarfs, such emissions are an indication of intrinsic stellar activity. But in dwarf carbon stars they could be due either to activity induced by interactions with the white dwarf or directly to heating from the white dwarf [48]. To confirm the source of the Balmer line emission, further observations are needed.

We also checked the GALEX public archive for UV emission from the white dwarf companion of LSR J2105+2514. There are two exposures of 60 s each separated by 1 h in both filters (FUV and NUV) of the field from 2007, but there is no detection of signal at the position of the dwarf. This does not imply the absence of a white dwarf but rather that it may be too faint to detect, particularly given the short exposure times used.

6. Discussion

In this work, we presented for the first time (to our knowledge) the spectral classification, distance measurements, effective temperature, and log g estimates as well as H$\alpha$ emission and chromospheric activity measurements for 37 ultracool dwarfs of late-M and early-L spectral types.

The spectral types are consistent with ones determined either by photometric studies or by infrared spectra published in the literature for eight of our objects [40,41]. Our estimated effective temperatures also showed agreement within $2-3\sigma$ of recently published estimates and general trends for late-type M and early-L dwarfs [37,41,43]. The estimated log g = 5.0–5.5 values are consistent with field late-M and early-L dwarfs [37].

We also determined log(L_Hα/L_bol), a commonly used indicator for magnetic activity, for 29 of the dwarfs and 3$\sigma$ upper limits for the rest. We find that our values are typical for these spectral types (Figure 3 in Metodieva et al. [25]). The emission can also be variable on timescales from minutes to hours. Until recently, the most common explanation for the emission was chromospheric heating produced by reconnection of strong magnetic fluxes [51] (and references therein). In recent years, however, another possibility has been suggested—auroral radio emission [52].

Finally, we report for the first time Balmer line emission from the dwarf carbon star 2MASS J21051653+2514486 (LSR J2105+2514). The emission was found to be variable. But to be able to constrain the level and timescale of variability, as well as to determine the nature of the emission, further monitoring observations are needed.

All of the ultracool dwarfs are situated within 100 pc of the Sun (27 are within 50 pc) and may present interest for searches of exoplanets, including ones that are potentially habitable. Additional information, such as rotational periods, photometric and H$\alpha$ variability, and magnetic field strengths, would help to fully characterise these dwarfs and thus the space weather around them.

The present study contributes also to the overall characterisation of the stars in the solar vicinity. It benefits from the obvious synergy between large ground-based observing facilities (in this case GTC), existing ground- and space-based surveys (2MASS and WISE), and current flagship space missions (ESA Gaia). Further similar studies have the potential to greatly improve our understanding of the most populous objects in the solar neighbourhood and in stellar populations in general. For example, studies based on higher-resolution spectra, together with an explicit treatment of metallicity, would likely improve the robustness of our temperature estimates and provide indications of the ages of the dwarfs; further, photometric monitoring can provide rotational periods and additional activity indicators (flares, spots, etc.), which can shed additional light on the rotation–activity relation.