V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode
by
Vitaly P. Goranskij
1,2,*, 
Elena A. Barsukova
2,
Aleksandr N. Burenkov
2,
Natalia V. Metlova
1,
Alla V. Zharova
1 and
Ilya A. Yakunin
2 1
Sternberg Astronomical Institute, Moscow State University, Universitetski pr., 13, Moscow 119992, Russia
2
Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhys, Zelenchukskiy reg., Karachai-Cherkessian Republik 369167, Russia
*
Author to whom correspondence should be addressed.
Galaxies 2025, 13(3), 59; https://doi.org/10.3390/galaxies13030059
Submission received: 5 February 2025
/
Revised: 14 April 2025
/
Accepted: 23 April 2025
/
Published: 14 May 2025
(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)
Abstract
The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We employed medium- and high-dispersion spectroscopy, along with multicolor photometry, to study this event in detail. Over 6 years, beginning in 2018, we observed the cessation of disk accretion, the filling of the accretor’s Roche lobe, and the subsequent formation of an A-type star within it. The pulsating envelope of the M giant donor was transferred to the accretor down to its base. Thus, the products from the hydrogen-burning layer appeared on the donor’s surface, and a flash of an emission-line spectrum enriched with s-process elements was detected. We discuss discrepancies between theoretical predictions and observations, as well as other phenomena potentially related to dynamical mass transfer.
1. Introduction
In the case of stable mass transfer between components of a close binary system, gas flows from the donor star through the inner Lagrange point, forming an accretion disk around the accretor star. However, if the radius of the donor star’s Roche lobe decreases as mass is transferred from a more massive star to a less massive one, the donor star overfills its Roche lobe by an increasingly larger amount, leading to runaway mass transfer on a dynamical timescale [1]. This occurs when the donor star is a giant or supergiant with a convective envelope. Otherwise, if the donor star loses mass, its radius shrinks faster than the radius of its Roche lobe, preventing the transfer. In the dynamical mode, the mass transfer becomes unstable, and the transferred material begins to expand, eventually filling and overfilling the accretor’s Roche lobe. This process results in the formation of a common-envelope (CE) system. The two components spiral inward until the CE is ejected [2]. These theoretical concepts can be tested by an event observed in the symbiotic binary V694 Mon (MWC 560), which began in 2018 and attracted the attention of the broader astronomical community. This system consists of an M3–M6 III giant, which pulsates semi-regularly with a period of 331–339 days [3,4], and an accreting white dwarf. The radius of this red giant is estimated to be approximately 140 solar radii [5]. The system is observed “face-on”, meaning the axis of its orbit is inclined by less than 30° relative to the line of sight. In the spectra of V694 Mon, blue-shifted absorption components appear in Balmer lines, expanding to radial velocities of up to −6500 km s−1 [6]. This phenomenon has been explained by the collimated ejection of matter directed along the line of sight. The photometric history of the system shows a gradual increase in brightness, with regular outbursts repeating with a period of 1860–1943 days [4,7,8]. These outbursts have been attributed either to the precession of jets, similar to those in SS 433 [9], or to orbitally-modulated activity near periastron in an elliptical orbit [5,7], with the eccentricity estimated to be between 0.68 and 0.82 [5]. Additionally, the star exhibits high-amplitude flickering on timescales ranging from minutes to hours.
2. Observations and Methods
We collected photometric data spanning 125 years (1899–2024), including photographic (eye estimates and microphotometry), photoelectric, and CCD in the systems by Johnson UBVRI, Cousins RI, and Straižys WBVR. The observations were carried out using Lyuty’s UBV photometer, CCD UBVRI photometers at the 60-cm Zeiss reflectors (Carl Zeiss, Jena, GDR), and the Maksutov meniscus 50-cm telescope (LOMO, Leningrad, USSR) at the Crimean station of the Moscow University. Additional observations were obtained using Kornilov’s four-channel WBVR photometer at the 1-m Zeiss reflector at the Tien-Shan Astronomical Observatory in Kazakhstan, and CCD UBVRI photometers at a similar 1-m Zeiss reflector of SAO RAS. The measuring equipment was manufactured at Moscow University and SAO. Two of the earliest images of V694 Mon, dated 6 February 1899 (Figure 1a) and 19 March 1900, were found in the Moscow plate collection of Sternberg Astronomical Institute taken with a 9.7 cm Steinheil (München, Germany) lens (f = 64 cm) in the Moscow Observatory. The images are located near the edge of the plates. These plates were processed electronically.
Additionally, we used published data, primarily from [4,7,10], as well as other less numerous sources. The total number of single- and multicolor observations is 4789, spanning the time range JD 2,414,692–2,460,700. This is a heterogeneous data set. Note that the early Moscow and Sonneberg data may be incomplete due to the lack of brightness estimates below the 13th magnitude, as they were limited by the sensitivity of the photographic plates. The complete light curve in the B band is shown in Figure 2.
The spectroscopic data include 53 medium-resolution spectra (3–14 Å), obtained between 2011 and 2024 with the 1-m Zeiss/UAGS at SAO RAS and the 6-m BTA/SCORPIO [11]. We have also three high-resolution spectra taken with BTA/MSS between 2019 and 2023. The MSS spectra were calibrated using a thorium-argon lamp. The resolution is 0.29 Å, the dispersion is 0.121 Å/px, and the wavelength scale is determined with an RMS accuracy of 0.005 Å, which matches the precision of modern spectral line catalogs for chemical elements.
3. Results
The analysis of photometric observations obtained before May 2018 confirms the previously reported periodicities. To refine these values, long-term brightening trends were removed from the light curves, and a period search was conducted using the residuals. For the outburst cycle, we determined a refined period of 1914 ± 15 days. The phased light curve is shown in Figure 3a. The pulsation period was refined using data from the Cousins I-band data (λ = 0.79 μm) and the phase-dispersion minimization method [12]. The resulting light curve, plotted with the period of 333.7 ± 0.2 days, exhibits a clear double-wave structure (Figure 3b). The 1914-day period is also evident in this band. Samples of flickering recorded on the nights of 6 October and 13 November 1996 are shown in Figure 4. These data were obtained with Kornilov’s four-channel photometer, which splits the light beam through dichroic filters. These observations are synchronous across the four filters of the Straižys WBVR system and were collected using the 1-m Zeiss telescope at the Tien-Shan Observatory in Kazakhstan. The highest flickering amplitude is observed in the ultraviolet W filter, whose response lies entirely shortward the Balmer limit.
In Figure 5a, we present medium-resolution blue spectra of V694 Mon obtained between 2016 and 2018. These spectra are typical for this system, showing variable absorption components in the Balmer lines, likely produced by the jet, and strong red emission wings, possibly formed by the counter-jet. As reported in [13], the absorption components of these lines were detached, meaning there was a segment of undistorted continuum between the absorption and emission components. This suggests that the radial velocity dispersion within the jet was lower than the jet’s bulk velocity. However, this effect was not seen in 2017–2018. The spectra are rich in metal emission lines, with the strongest ones exhibiting P Cygni profiles. Figure 5b shows red spectra, highlighting Hα profiles and their surroundings. The earliest Hα profile, from January 2011, displays the strongest absorption component, extending blueward up to −4500 km s−1 —the highest velocity observed in our dataset. In 2017–2018, jet velocities did not exceed −2000 km s−1, and the absorption profiles were no longer detached.
A dramatic change in the star’s behavior occurred in October 2018. (1) Flickering activity ceased. (2) The system brightened by 1 magnitude approximately 960 days before the end of the 1914-day cycle. (3) High-velocity absorption components in Balmer lines disappeared, signaling a halt in disk accretion and jet formation [14]. The accretion disk was destroyed [15]. Photometric changes in the V filter are shown in Figure 6 (top), and corresponding spectral changes in Figure 7. After October 2018, the amplitude of light variations from the M giant in the Ic band decreased; however, the overall brightness continued rising with increased scatter (Figure 6, bottom). Post-2018 Ic-band data were analyzed for pulsations, with efforts to minimize systematic discrepancies between instruments. The periodograms reveal the most prominent peaks at periods of 321, 168, and 84 days, with amplitudes around 0.15 mag. These periods are consistent with pre-2018 values. Despite period ratios near 1:2, the post-2018 light curves are single-wave, likely due to limited data coverage. Observations continue.
Spectra obtained after the 2018 event reveal the progressive filling of the white dwarf’s Roche lobe with the matter lost by the donor, leading to the formation of an A-type star photosphere [16]. From 2018 to 2020, the emission lines weakened, while P Cygni-type absorption components in Balmer and metal lines became stronger. This suggests an increased density of matter in the accretor’s Roche lobe. On 21 November 2021, we recorded the spectrum of an A-type star with hypertrophied absorption lines, indicating the presence of a substantial atmosphere. This medium-resolution spectrum, and those that followed, showed increased scatter—especially in the blue. The high-resolution spectrum obtained with the BTA/MSS on 17 February 2022, revealed this scatter to be due to a weak emission-line flare. We identified lines of Y, Zr, Nb, Ba, La, Ce, Nd, and other exotic elements, with cerium dominating (Figure 8, Table A1). These lines appeared at a heliocentric velocity of +39 km s−1. They were narrow, with a full width at half maximum (FWHM) of about 0.3 Å, indicating that their profiles were unresolved. A spectrum fragment (Figure 9) illustrates the accuracy of these identifications with the database wavelength. These elements are produced by s-process (slow neutron capture). We interpret the appearance of this spectrum as a manifestation of the lower layer of the discarded donor’s envelope.
As shown in Table A1, a significant number of these lines deviate from catalog values by several hundredths of an angstrom. Notably, none match precisely. Given the high measurement accuracy, these deviations are significant. We suggest these are transitions of short-lived isotopes produced in the s-process, while catalogs list lines for stable isotopes only. We also considered the possibility that the shifted lines could arise from another emission region with a different velocity. One alternative explanation involves O II lines at +17 km/s (see table notes).
Since 2023, we have observed the newly formed A star in the accretor’s Roche lobe, as it reaches hydrodynamic equilibrium. We classified it as an A4 I-type star. Figure 7b (bottom) compares its blue spectrum to that of standard star SAO 12096 (A4 I). A comparison of the full calibrated spectrum of V694 Mon with the standard star is given in Figure 10.
4. Discussion
The mass transfer event in V694 Mon represents an unprecedented case in astrophysics: the emergence of an A4 I type star with an apparent age of only six years. Based on the known distance to V694 Mon of d = 2.36 Kpc (π = 0.424 ± 0.035 mas, Gaia DR3 [17]) and recent averaged photometric measurements between 4 December 2023 and 5 April 2024—UBV = (8.54, 8.59, 8.34 mag)—along with a color excess of E(B − V) = 0.17 mag and visual extinction AV = 0.52 mag, we derive the stellar parameters Mbol = −4.02 ± 0.03 mag and Teff = 8600 K. The color excess E(B − V) was estimated using a two-color diagram for luminosity class I stars. A comparable bolometric luminosity, albeit for a cooler A7 II/I star with Teff =7200 K, was previously reported in [18]. With these parameters, the A-type star is now as luminous as a post-main-sequence A-type star with a mass of 6.5 M⊙. This marks a significant departure from earlier classifications, where the accretor was believed to be a white dwarf.
The alternative hypothesis suggested that the changes experienced in 2018 are due to V694 Mon transitioning from the accretion-only state (dominating since the object discovery) to the non-explosive, thermal-equilibrium hydrogen burning on the WD surface [18,19]. The bolometric luminosity of such stable burning is estimated to be approximately 3500 L⊙, corresponding to a white dwarf mass of 0.6 M⊙. The authors of [19] put forward a hypothesis that the emission from a burning shell surrounding the white dwarf could mimic the spectrum of a normal giant star.
To evaluate the nature of the 2018 event in V694 Mon, it is crucial to consider the characteristics of its donor, an M-type giant. Medium-resolution infrared spectroscopy during the 2016 outburst [20] revealed a strong photospheric absorption feature of ZrO, confirming the donor as an S-type star—that is, a giant enriched in s-process elements. As a result of the event that began in 2018, the deep layers of the S star donor’s envelope were exposed, triggering an enhancement in the s-process emission lines—clear evidence of a dynamical mass transfer episode originating from the donor.
However, it is well established that disk accretion in symbiotic novae—similar to classical novae—typically results in a thermonuclear explosion of the hydrogen shell on the white dwarf surface. This high-energy event leads to the expansion or even ejection of the shell. For instance, the symbiotic nova V4368 Sgr underwent an outburst with an amplitude of 11.24 mag [21], exceeding the typical values observed in classical novae. At peak brightness, it exhibited an sgF5 spectrum with prominent emission lines, and its luminosity increased by a factor of 31,000.
In contrast, no envelope expansion was observed in V694 Mon. During the 2018 event, its brightness increased by only a factor of 50, and over the past 125 years, by a factor of 100. The star’s slow brightening over roughly 70 years suggests that hydrogen burning on the white dwarf surface may have been occurring prior to 2018. The absolute magnitude measured from the 1899 photographic plate, MB = +1 mag (65.5 L⊙), indicates that the system was already more luminous than expected from a white dwarf, its accretion disk, and the contribution from an M-type donor alone. This suggests that stellar thermonuclear burning was already occurring at that time.
Then, the 2018 event may be triggered by the dynamical mass transfer of several solar masses, or alternatively, by accretion onto a more massive compact object that is not a white dwarf. It is also possible that both factors contributed. In this context, the 2018 event represents a second major episode of mass exchange in the system, with the accretor being a massive subdwarf—possibly the remnant core of a once massive star.
As predicted by theory, a convective envelope in the donor star is essential to enable the dynamical mass transfer. Red giants in the post-AGB stage exhibit s-process spectral lines in absorption in their photospheres due to regular “thermal pulses” that dredge up synthesized material from the stellar interior. The same s-process elements—whose emission lines are detected in V694 Mon—were also found and studied in absorption in the post-AGB star GSC 04050−02366 [22]. However, these lines were not detected in our first high-resolution spectrum obtained on 8 March 2019. The narrow profiles of these lines in the subsequent two spectra suggest that they are formed on the surface of the donor, not in a wind or in an envelope of an A-type star. It is known that the donor’s envelope was radially pulsating. However, the interaction between pulsations, convection, and accretion under dynamical mass transfer remains poorly understood. We propose an additional factor to support mass transfer. The donor, possessing a massive core, may not have had sufficient time to reach hydrodynamic equilibrium, to shrink its radius quickly enough to suppress runaway transfer.
The s-process (slow neutron-capture process) involves a sequence of neutron captures by heavy atomic nuclei in regions of thermonuclear burning. This process increases the atomic mass of the nuclei. Following neutron capture, β-decay can occur, in which the nucleus emits an electron raising its atomic number and forming a new element. The s-process is responsible for the synthesis of both long- and short-lived heavy elements and their isotopes. If material enriched by the s-process is rapidly removed from the burning region, its spectrum may change abruptly. This effect was observed in our high-resolution spectrum on 31 October 2023: numerous narrow emission lines vanished, including those of Zr II with Ba II (Figure 8, bottom).
5. Conclusions
Mass transfer in the dynamical mode may last for approximately three years, during which the donor’s envelope is fully transferred onto the accretor. At such a high accretion rate, the hydrodynamic equilibrium of the donor cannot be restored to prevent runaway transfer. In systems with a massive donor, the center of mass of the system is located within the donor itself. As a result, matter overflowing the donor’s Roche lobe accretes onto the low-mass companion from all directions—not solely through the inner Lagrange point and the orbital plane. Simultaneously, the inner Lagrange point sinks deeper into the donor’s volume as its Roche lobe radius decreases. These effects disrupt the accretion disk, halting disk-mediated accretion and rapidly filling the accretor’s Roche lobe with a stellar material. Our observations indicate that once disk accretion ceased, a normal star of spectral type A4 I formed within the accretor’s Roche lobe.
The concept of dynamical mass transfer may also shed light on other astrophysical phenomena. For example, it can help explain the relativistic binary system SS 433, where dynamical mass transfer happens from an A4–7 III–I donor onto a magnetic neutron star, potentially forming a convective star with a neutron core (Thorne-Żytkow object). This extremely hot star has a surface temperature of about 1.5 million K [23]. Similarly, the recurrent nova M31 2008-12a with a red giant donor, which undergoes outbursts every 174 days followed by an SSS phase [24], or the enigmatic ULX and SN2010da system in the galaxy NGC 300, comprising a B[e] star and a red supergiant [25], may also be linked to such mass transfer events. Furthermore, dynamical mass transfer may be a key driver behind phenomena such as stellar mergers, Intermediate Luminosity Optical Transients (ILOTs), and some types of supernovae.
Author Contributions
Conceptualization, writing, analysis, photometry, spectroscopy, observations, data reducing, software, V.P.G.; spectroscopy, data reducing, editing, E.A.B.; spectroscopy, observations, A.N.B.; photometric observations, N.V.M.; photometric observations, A.V.Z.; high-resolution spectroscopy, observations, I.A.Y. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Data Availability Statement
Our observational data are available in the following archive: https://relay.sao.ru/jet/~bars/V694Mon-ARCHIVE/, in files V694-SPECTRA.zip for the spectral data and VBURI-PTM.zip for the photometry. URL accessed on 22 April 2025.
Acknowledgments
This study was conducted under the state assignment of Lomonosov Moscow State University. Observations with the SAO RAS telescopes were supported by the Ministry of Science and Higher Education of the Russian Federation. The renovation of telescope equipment is currently provided within the national project “Science and Universities”. V.P.G. is grateful to the American Astronomical Society for support of observations at the Tien-Shan Astronomical Observatory (Kazakhstan) in 1994–1996. We thank A. S. Vinokurov for the spectral observations carried out for us with the Russian 6-m telescope BTA and the SCORPIO-2 multimode focal reducer.
Conflicts of Interest
The authors declare no conflicts of interests.
Appendix A
Table A1.
Identification of lines in the weak emission spectrum on 17 February 2022.
| Atom/ Ion | Obs. λ (Å) | Cat. λ (Å) | EW (Å) | Data Base | Remarks |
|---|---|---|---|---|---|
| La II | 4429.89 | 4429.90 | −0.07 | Co93 | 4429.90 |
| Zr II | 4440.45 | 4440.45 | −0.03 | Co93 | |
| C IV | 4441.78 | 4441.74 | −0.03 | NIST | ? |
| Ho I | 4444.62 | 4444.63 | −0.03 | NIST | |
| Nd II | 4446.38 | 4446.387 | −0.01 | NIST | 4446.37 weak |
| Ce II | 4449.33 | 4449.336 | −0.05 | Co93 | 4449.33 |
| Nd II | 4451.57 | 4451.566 | −0.02 | Co93 | |
| Zr II | 4454.79 | 4454.80 | −0.05 | Co93 | |
| Zr II | 4457.45 | 4457.42 | −0.02 | Co93 | |
| Zr II | 4461.25 | 4461.22 | −0.02 | Co93 | |
| Nd II | 4462.96 | 4462.985 | −0.01 | Co93 | 4462.90 weak |
| Ti II | 4469.17 | 4469.16 | −0.03 | Co93 | |
| Fe II | 4472.95 | 4472.921 | −0.01 | Co93 | |
| Ce II | 4479.36 | 4479.35 | −0.03 | Co93 | 4479.32 blend |
| Ce II | 4486.89 | 4486.909 | −0.04 | Co93 | 4486.86 |
| Ti II | 4493.52 | 4493.523 | −0.03 | NIST | |
| Zr II | 4494.44 | 4494.41 | −0.03 | Co93 | 4494.47 |
| Zr II | 4495.41 | 4495.44 | −0.02 | Co93 | |
| Zr II | 4497.00 | 4496.96 | −0.04 | Co93 | 4497.04 abs PCyg |
| V II | 4518.36 | 4518.38 | −0.04 | Co93 | 4518.19 abs PCyg |
| La II | 4526.11 | 4526.12 | −0.03 | NIST | 4526.06 |
| Ce II | 4527.34 | 4527.348 | −0.05 | Co93 | 4527.30 |
| V II | 4528.53 | 4528.51 | −0.04 | Co93 | blend |
| Cr I | 4529.53 | 4529.50 | −0.03 | NIST | PCyg? |
| Ce II | 4539.75 | 4539.755 | −0.06 | NIST | 4539.72 |
| Ti II | 4544.04 | 4544.009 | −0.02 | Co93 | 4543.97 abs PCyg |
| Ti II | 4545.15 | 4545.144 | −0.04 | Co93 | |
| Ce II | 4551.27 | 4551.297 | −0.03 | Co93 | 4551.24 |
| Ti II | 4552.32 | 4552.289 | −0.04 | NIST | |
| Ba II | 4554.10 | 4554.033 | −0.07 | Co93 | 4553.29 abs PCyg |
| Ce II | 4560.29 | 4560.280 | −0.01 | Co93 | |
| Ce II | 4562.31 | 4562.360 | −0.01 | Co93 | |
| Ce II | 4562.32 | 4562.36 | −0.01 | NIST | |
| V II | 4564.61 | 4564.592 | −0.02 | Co93 | |
| La II | 4574.83 | 4574.87 | −0.04 | Co93 | 4574.84 blend |
| Fe II | 4576.40 | 4576.405 | −0.02 | Co93 | 4576.22 abs PCyg |
| La II | 4580.06 | 4580.05 | −0.01 | Co93 | 4579.93 abs PCyg blend FeII |
| La II | 4613.35 | 4613.38 | −0.02 | Co93 | 4613.30 O II (at 16–17 km/s)? |
| Zr II | 4613.94 | 4613.95 | −0.02 | Co93 | 4613.92 O II (at 16–17 km/s)? |
| Fe II | 4620.49 | 4620.513 | −0.01 | Co93 | 4620.40 abs PCyg |
| Ti II | 4636.34 | 4636.345 | −0.01 | Co93 | |
| Fe II | 4657.22 | 4656.974 | −0.02 | Co93 | 4656.91 abs PCyg blend TiII |
| Zr II | 4661.77 | 4661.78 | −0.02 | Co93 | |
| La II | 4662.50 | 4662.51 | −0.02 | Co93 | |
| La II | 4663.76 | 4663.76 | −0.01 | Co93 | |
| Fe II | 4670.45 | 4670.17 | −0.02 | Co93 | abs PCyg |
| Y II | 4682.31 | 4682.321 | −0.07 | Co93 | 4682.26 |
| Ce II | 4684.56 | 4684.605 | −0.02 | Co93 | 4684.51 |
| Nb I | 4685.16 | 4685.14 | −0.01 | NIST | ZrII (4685.19)? |
| unid. | 4691.25 | −0.01 | O II (4691.416, 17 km/s)? | ||
| Dy II | 4698.68 | 4698.68 | −0.01 | NIST | |
| Ga II | 4706.48 | 4706.51 | −0.01 | NIST | Nd II (4706.542)? |
| Pr II | 4707.51 | 4707.541 | −0.01 | Co93 | |
| Ti II | 4708.68 | 4708.663 | −0.03 | Co93 | 4708.55 abs PCyg |
| Ce II | 4713.98 | 4714.00 | −0.01 | NIST | 4713.92 |
| Nd II | 4715.60 | 4715.589 | −0.01 | NIST | |
| Nd II | 4724.34 | 4724.35 | −0.01 | NIST | |
| unid. | 4725.02 | −0.01 | |||
| La II | 4728.40 | 4728.41 | −0.02 | Co93 | 4728.35 blend |
| Fe II | 4731.51 | 4731.439 | −0.01 | Co93 | 4731.34 abs PCyg |
| Zr II | 4739.43 | 4739.48 | −0.01 | NIST | 4739.40 blend +Ce II 4739.53 |
| La II | 4740.26 | 4740.27 | −0.03 | Co93 | 4740.18 |
| C II | 4744.90 | 4744.90 | −0.02 | Co93 | Pr II 4744.925? |
| V I | 4747.08 | 4747.099 | −0.02 | NIST | 4740.06 |
| La II | 4748.71 | 4748.73 | −0.01 | NIST | 4748.66 |
| Ce II | 4757.83 | 4757.842 | −0.01 | Co93 | 4757.49 |
| Er II | 4759.57 | 4759.65 | −0.01 | NIST | ? |
| Ti II | 4762.79 | 4762.778 | −0.02 | NIST | abs PCyg Zr I 4762.78? |
| Nd II | 4763.90 | 4763.865 | −0.03 | Co93 | abs PCyg TiII 4763.8833? |
| Ti II | 4764.53 | 4764.535 | −0.02 | Co93 | |
| Ti II | 4780.04 | 4779.986 | −0.02 | Co93 | 4779.89 PCyg |
| Y II | 4786.59 | 4786.58 | −0.03 | Co93 | |
| F II | 4789.48 | 4789.45 | −0.01 | NIST | |
| Hf I | 4792.44 | 4792.45 | −0.01 | NIST | blend |
| unid. | 4793.19 | −0.01 | |||
| Ti II | 4798.54 | 4798.535 | −0.04 | Co93 | 4798.53 PCyg |
| La II | 4809.00 | 4809.00 | −0.01 | Co93 | 4808.94 |
| Nd II | 4811.32 | 4811.343 | −0.01 | Co93 | 4811.25 abs |
| Fe II | 4814.52 | 4814.544 | −0.02 | NIST | 4814.47? |
| Sm II | 4815.80 | 4815.808 | −0.01 | Co93 | |
| Nb I | 4816.38 | 4816.38 | −0.01 | NIST | |
| NdII | 4820.32 | 4820.336 | −0.01 | Co93 | 4820.24 |
| Y II | 4823.28 | 4823.304 | −0.03 | NIST | |
| Nd II | 4825.56 | 4825.482 | −0.03 | NIST | 4825.42? |
| Hf I | 4828.53 | 4828.580 | −0.01 | NIST | |
| Ta I | 4832.21 | 4832.185 | −0.01 | NIST | 4832.21 blend +NdII 4832.28 |
| Fe II | 4833.22 | 4833.21 | −0.01 | Co93 | PCyg |
| La II | 4840.02 | 4840.02 | −0.01 | Co93 | |
| Sm II | 4844.21 | 4844.208 | −0.01 | Co93 | |
| Ti II | 4849.16 | 4849.18 | −0.04 | Co93 | 4849.04 |
| Y II | 4854.90 | 4854.87 | −0.05 | Co93 | abs PCyg Fe I 4854.89? |
| Ti II | 4865.62 | 4865.620 | −0.04 | Co93 | |
| Fe I | 4871.32 | 4871.323 | −0.02 | Co93 | abs PCyg |
| Ti II | 4874.04 | 4874.025 | −0.01 | Co93 | abs PCyg |
| Cr II | 4876.51 | 4876.48 | −0.01 | Co93 | 4876.38 abs PCyg |
| Y II | 4883.73 | 4883.69 | −0.06 | Co93 | abs PCyg |
| Y II | 4900.13 | 4900.13 | −0.01 | Co93 | 4899.934 Ba II blend |
| Ti II | 4911.23 | 4911.205 | −0.01 | Co93 | 4911.34 LaII PCyg blend |
| Cs II | 4914.23 | 4914.238 | −0.02 | NIST | |
| unid. | 4919.63 | −0.03 | |||
| La II | 4920.94 | 4920.98 | −0.06 | Co93 | 4920.92 |
| La II | 4921.77 | 4921.80 | −0.06 | Co93 | 4921.71 |
| Fe I | 4927.47 | 4927.42 | −0.01 | Co93 | ? |
| Ba II | 4934.10 | 4934.086 | −0.04 | Co93 | 4933.91 abs PCyg |
| La II | 4970.35 | 4970.39 | −0.02 | Co93 | 4970.38 |
| Ce II | 4971.47 | 4971.475 | −0.01 | Co93 |
Description of the Table. Column 1: Chemical element and its ionization level for the emission lines identified in the spectrum on 17 February 2022 (unid. = unidentified). Column 2: Observed wavelength. Column 3: Catalogue wavelength. Column 4: Equivalent width of line (Å). 5: Databases: Co93—Coluzzi Database [26]; NIST Database [27]. Remarks: The digital values of the wavelength and the equivalent width estimates give the same values as this line measured in the spectrum on 31 October 2023. abs—absorption component in this line if present. PCyg—P Cygni type profile. Some data are given in case of alternative interpretation. ?—the identification is doubtful.
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Figure 1.
Photographic image of V694 Mon at the end of the 19th century (a), and CCD image in the 21st century (b). The images are marked with circles. The brightness of the star increased by a factor of 100.
Figure 1.
Photographic image of V694 Mon at the end of the 19th century (a), and CCD image in the 21st century (b). The images are marked with circles. The brightness of the star increased by a factor of 100.
Figure 2.
The 125-year light curve of V694 Mon in the B band. The historic early observations are marked with circles.
Figure 2.
The 125-year light curve of V694 Mon in the B band. The historic early observations are marked with circles.
Figure 3.
Phased light curves of periodic components in the light variations of V694 Mon. (a) The outbursts in the B band, and (b) The pulsations in the IC band.
Figure 3.
Phased light curves of periodic components in the light variations of V694 Mon. (a) The outbursts in the B band, and (b) The pulsations in the IC band.
Figure 4.
Samples of V694 Mon flickering recorded over the nights in 1996 simultaneously in WBVR filters (from top to bottom).
Figure 4.
Samples of V694 Mon flickering recorded over the nights in 1996 simultaneously in WBVR filters (from top to bottom).
Figure 5.
Spectra of V694 Mon before May 2018. Blue region (a) and red region in the vicinity of Hα emission (b). Dates increase from top to bottom.
Figure 5.
Spectra of V694 Mon before May 2018. Blue region (a) and red region in the vicinity of Hα emission (b). Dates increase from top to bottom.
Figure 6.
Photometric changes in light curves in 2018. The upper V band curve shows the cessation of flickering and the brightness jump. The lower IC-band light curve shows light changes in the envelope of the M giant in the course of mass transfer.
Figure 6.
Photometric changes in light curves in 2018. The upper V band curve shows the cessation of flickering and the brightness jump. The lower IC-band light curve shows light changes in the envelope of the M giant in the course of mass transfer.
Figure 7.
Spectroscopic changes after October 2018. (a,b) The blue region. From October 2018 to December 2020, there is a disappearance of the high-velocity component in hydrogen lines, as well as strengthening absorption in P Cygni profiles of hydrogen and metal lines. From November 2021 to March 2022, the formation of an A-type star with a low-density atmosphere and hypertrophied metal lines is observed, followed by the attainment of hydrodynamic equilibrium in an A4 I star. The last spectrum in the (b) panel is that of an A4 I standard. (c) The red region. Evolution of the spectrum in the Hα profile and its vicinity. The Si II doublet indicates a mid-A spectrum. Note the persistent weak Hα emission.
Figure 7.
Spectroscopic changes after October 2018. (a,b) The blue region. From October 2018 to December 2020, there is a disappearance of the high-velocity component in hydrogen lines, as well as strengthening absorption in P Cygni profiles of hydrogen and metal lines. From November 2021 to March 2022, the formation of an A-type star with a low-density atmosphere and hypertrophied metal lines is observed, followed by the attainment of hydrodynamic equilibrium in an A4 I star. The last spectrum in the (b) panel is that of an A4 I standard. (c) The red region. Evolution of the spectrum in the Hα profile and its vicinity. The Si II doublet indicates a mid-A spectrum. Note the persistent weak Hα emission.
Figure 8.
High-resolution spectra obtained with the Russian 6 m telescope BTA with the Main Stellar Spectrograph (MSS). The brightest s-process emission lines are identified.
Figure 8.
High-resolution spectra obtained with the Russian 6 m telescope BTA with the Main Stellar Spectrograph (MSS). The brightest s-process emission lines are identified.
Figure 9.
Fragment of high-resolution BTA/MSS spectrum showing s-process Zr II, Ce II, and Nd II emission. The Ti II line may have a dual origin, combining narrow emission from the s-process and wider absorption from the A-type star photosphere. That duality may explain why some narrow lines have “P Cygni profiles”.
Figure 9.
Fragment of high-resolution BTA/MSS spectrum showing s-process Zr II, Ce II, and Nd II emission. The Ti II line may have a dual origin, combining narrow emission from the s-process and wider absorption from the A-type star photosphere. That duality may explain why some narrow lines have “P Cygni profiles”.
Figure 10.
Comparison of the calibrated spectrum of V694 Mon with the A4 I spectroscopic standard HD 12096. The spectrum of V694 Mon shows the presence of weak Hα emission.
Figure 10.
Comparison of the calibrated spectrum of V694 Mon with the A4 I spectroscopic standard HD 12096. The spectrum of V694 Mon shows the presence of weak Hα emission.
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MDPI and ACS Style
Goranskij, V.P.; Barsukova, E.A.; Burenkov, A.N.; Metlova, N.V.; Zharova, A.V.; Yakunin, I.A. V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode. Galaxies 2025, 13, 59. https://doi.org/10.3390/galaxies13030059
AMA Style
Goranskij VP, Barsukova EA, Burenkov AN, Metlova NV, Zharova AV, Yakunin IA. V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode. Galaxies. 2025; 13(3):59. https://doi.org/10.3390/galaxies13030059
Chicago/Turabian StyleGoranskij, Vitaly P., Elena A. Barsukova, Aleksandr N. Burenkov, Natalia V. Metlova, Alla V. Zharova, and Ilya A. Yakunin. 2025. "V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode" Galaxies 13, no. 3: 59. https://doi.org/10.3390/galaxies13030059
APA StyleGoranskij, V. P., Barsukova, E. A., Burenkov, A. N., Metlova, N. V., Zharova, A. V., & Yakunin, I. A. (2025). V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode. Galaxies, 13(3), 59. https://doi.org/10.3390/galaxies13030059
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