Search Results (5)

This article presents the results of long-term spectral studies of the unusual Herbig Ae star V1295 Aql. The variability in the spectrophotometric parameters of the emission and absorption components of the hydrogen lines Hα, Hβ, and D Na I—and of various metallic lines—is studied. The periodic variability in these parameters over a period of 51.7 days, recently established using mean longitudinal magnetic field measurements, is confirmed. The obtained average value of the radial velocities of metal absorption lines is Vr = 0.75 ± 1.85 km/s. According to our long-term observations, the radial velocities of the star do not change over time. This indicates the absence of a close stellar-mass companion. Full article

We present results of the investigation of the star LkHα 324/B, which belongs to the starforming dark cloud LDN 988. Based on high resolution spectroscopy, we determined its fundamental parameters as $T_{\mathrm{eff}}=\mathrm{11,175}\pm 130$ K, $\log (L_{*}/L_{\odot })=1.87\pm 0.07$. According to these parameters, we found that LkHα 324/B is a pre-main sequence star with mass $M\approx 3M_{\odot }$ and age $t\approx 2.9$ Myr. Recently, it underwent the phase of actively accreting the Herbig Ae/Be star, but accretion has now ceased in the LkHα 324/B system. This is consistent with the fact that the star is surrounded by a circumstellar disk with an inner cavity, as was determined from its spectral energy distribution. Our analysis revealed the peculiar abundance pattern of LkHα 324/B typical to those of magnetic Ap stars. It possesses mild underabundance of the light elements and excess up to ∼2–4 dex (in comparison to the Sun) of the iron peak and rare earth elements. We found no evidence for abrupt vertical abundances gradients in the lines forming the region of the LkHα 324/B atmosphere, in agreement with the results of the theoretical diffusion calculations in this temperature domain. From the intensification of the magnetically sensitive lines, we deduced that LkHα 324/B probably hosts a global magnetic field of $⟨B⟩\approx 3.5$ kG strength. We suppose that the stabilizing role of this field favored the elements’ separation by diffusion before the star reached the main sequence. Full article

At the time of stellar evolution, young stellar objects go through processes of increased activity and instability. Star formation takes place in several stages during which the star accumulates enough mass to initiate thermonuclear reactions in the nucleus. A significant percentage of the mass of Sun-like stars accumulates during periods of increased accretion known as FUor outbursts. Since we know only about two dozen stars of this type, the study of each new object is very important for our knowledge. In this paper, we present data from photometric monitoring on a FUor object V2493 Cyg discovered in 2010. Our data were obtained in the optical region with BVRI Johnson–Cousins set of filters during the period from November 2016 to February 2021. The results of our observations show that during this period no significant changes in the brightness of the star were registered. We only detect variations with a small amplitude around the maximum brightness value. Thus, since 2013 V2493 Cyg remains at its maximum brightness, without a decrease in brightness. Such photometric behavior is not typical of other stars from FUor type. Usually, the light curves of FUors are asymmetrical, with a very rapid rise and gradual decline of the brightness. V2493 Cyg remains unique in this respect with a very rapid rise in brightness and prolonged retention in maximum light. Our period analysis made for the interval February 2013–February 2021 reveals a well-defined period of 914 ± 10 days. Such periodicity can be explained by dust structures remaining from star formation in orbit around the star. Full article

We describe four different approaches for the detection of magnetospheric accretion among Herbig Ae/Be stars with accretion disks. Studies of several unique objects have been carried out. One of the objects is the Herbig Ae star HD 101412 with a comparatively strong magnetic field. The second is the early-type Herbig B6e star HD 259431. The existence of a magnetosphere in these objects was not recognized earlier. In both cases, a periodicity in the variation of some line parameters, originating near the region of the disk/star interaction, has been found. The third object is the young binary system HD 104237, hosting a Herbig Ae star and a T Tauri star. Based on the discovery of periodic variations of equivalent widths of atmospheric lines in the spectrum of the primary, we have concluded that the surface of the star is spotted. Comparing our result with an earlier one, we argue that these spots can be connected with the infall of material from the disk onto the stellar surface through a magnetosphere. The fourth example is the Herbig Ae/Be star HD 37806. Signatures of magnetospheric accretion in this object have been identified using a different method. They were inferred from the short-term variability of the He i $\lambda$5876 line profile forming in the region of the disk/star interaction. Full article

Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios. Full article