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Register a new userDiagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM Cephei
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UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting sporadic occultation of stellar light by circumstellar dust. GM,Cephei is such a UXor in the young ($sim4$~Myr) open cluster Trumpler,37, showing prominent infrared excess, emission-line spectra, and flare activity. Our photometric monitoring (2008--2018) detects (1)~an $sim$3.43~day period, likely arising from rotational modulation by surface starspots, (2)~sporadic brightening on time scales of days due to accretion, (3)~irregular minor flux drops due to circumstellar dust extinction, and (4)~major flux drops, each lasting for a couple of months with a recurrence time, though not exactly periodic, of about two years. The star experiences normal reddening by large grains, i.e., redder when dimmer, but exhibits an unusual blueing phenomenon in that the star turns blue near brightness minima. The maximum extinction during relatively short (lasting $leq 50$~days) events, is proportional to the duration, a consequence of varying clump sizes. For longer events, the extinction is independent of duration, suggestive of a transverse string distribution of clumps. Polarization monitoring indicates an optical polarization varying $sim3%$--8$%$, with the level anticorrelated with the slow brightness change. Temporal variation of the unpolarized and polarized light sets constraints on the size and orbital distance of the circumstellar clumps in the interplay with the young star and scattering envelope. These transiting clumps are edge-on manifestations of the ring- or spiral-like structures found recently in young stars with imaging in infrared of scattered light, or in submillimeter of thermalized dust emission.
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We present our photometric observations of GM Cep, a solar type variable in the young (~4 Myr) open cluster Trumpler 37. The star is known as a classical T Tauri star with a circumstellar disk and active accretion. GM,Cep was suspected to undergo an outburst, thus a candidate for an EXor-type variable. In our monitoring campaign observations in 2010-2011, GM Cep experienced a ~0.82 mag brightness decrease in the R band lasting for 39 days, and frequent, transient flare-like episodes with amplitude < 1 mag, each lasting for about 10 days. The brightening was accompanied with a bluer color, presumably arising from increased accretion activity. Interestingly, the star also turned bluer in the fading phase. Combining the AAVSO and literature data, we found a quasi-cyclic peroid of ~311 days for the fading event. A possible mechanism for the fading could be obscuration by a clump of dust around the star. We proposed that GM Cep therefore should be a UXor-type variable in the transition phase between grain coagulation and planetesimal formation process in the circumstellar disk.
New results from long-term optical photometric observations of the pre-main sequence star GM Cep from UX Orionis type are reported. During ongoing photometric monitoring of the GM Cep four deep minimums in brightness are observed. The collected multicolour photometric data shows the typical of UXor variables colour reversal during the minimums in brightness. Recent $BVRI$ photometric observations of GM Cep have been collected from November 2014 to October 2020.
V582 Aur is an FU Ori-type young eruptive star in outburst since $sim$1985. The eruption is currently in a relatively constant plateau phase, with photometric and spectroscopic variability superimposed. Here we will characterize the progenitor of the outbursting object, explore its environment, and analyse the temporal evolution of the eruption. We are particularly interested in the physical origin of the two deep photometric dips, one occurred in 2012, and one is ongoing since 2016. We collected archival photographic plates, and carried out new optical, infrared, and millimeter wave photometric and spectroscopic observations between 2010 and 2017, with high sampling rate during the current minimum. Beside analysing the color changes during fading, we compiled multiepoch spectral energy distributions, and fitted them with a simple accretion disk model. Based on pre-outburst data and a millimeter continuum measurement, we suggest that the progenitor of the V582 Aur outburst is a low-mass T Tauri star with average properties. The mass of an unresolved circumstellar structure, probably a disk, is 0.04 M$_{odot}$. The optical and near-infrared spectra demonstrate the presence of hydrogen and metallic lines, show the CO bandhead in absorption, and exhibit a variable H$alpha$ profile. The color variations strongly indicate that both the $sim$1 year long brightness dip in 2012, and the current minimum since 2016 are caused by increased extinction along the line of sight. According to our accretion disk models, the reddening changed from $A_V$=4.5 mag to 12.5 mag, while the accretion rate remained practically constant. Similarly to the models of the UXor phenomenon of intermediate and low-mass young stars, orbiting disk structures could be responsible for the eclipses.
Here we analyze the first simultaneous X-ray, ultraviolet, optical, infrared, and centimeter observations of a T Tauri star (TTS). We present three epochs of simultaneous Spitzer and VLA data of GM Aur separated by ~1 wk. These data are compared to previously published HST and Chandra observations from which mass accretion rates ($dot M$) and X-ray luminosities, respectively, were measured. The mid-infrared emission increases along with $dot M$, and we conclude that this is due to an increase in the mass in the inner disk. The cm emission, which probes the jet, also appears to increase as $dot M$ increases, and the changes in the cm flux are consistent with the variability in $dot M$ assuming the mass-loss rate is ~10% $dot M$. The 3 cm emission morphology also appears changed compared with observations taken three years previously, suggesting that for the first time, we may be tracking changes in the jet morphology of a TTS. The X-ray luminosity is constant throughout the three epochs, ruling out variable high-energy stellar radiation as the cause for the increases in the mid-infrared or cm emission. Tying together the multiwavelength variability observed, we conclude that an increase in the surface density in the inner disk resulted in more mass loading onto the star and therefore a higher $dot M$, which led to a higher mass-loss rate in the jet. These results stress the importance of coordinated multiwavelength work to better understand the star-disk-jet connection.
We analyze 3 epochs of ultraviolet (UV), optical and near-infrared (NIR) observations of the Taurus transitional disk GM Aur using the Hubble Space Telescope Imaging Spectrograph (STIS) and the Infrared Telescope Facility SpeX spectrograph. Observations were separated by one week and 3 months in order to study variability over multiple timescales. We calculate accretion rates for each epoch of observations using the STIS spectra and find that those separated by one week had similar accretion rates (~1E-8 solar masses/yr) while the epoch obtained 3 months later had a substantially lower accretion rate (~4E-9 solar masses/yr). We find that the decline in accretion rate is caused by lower densities of material in the accretion flows, as opposed to a lower surface coverage of the accretion columns. During the low accretion rate epoch we also observe lower fluxes at both far UV (FUV) and IR wavelengths, which trace molecular gas and dust in the disk, respectively. We find that this can be explained by a lower dust and gas mass in the inner disk. We attribute the observed variability to inhomogeneities in the inner disk, near the corotation radius, where gas and dust may co-exist near the footprints of the magnetospheric flows. These FUV--NIR data offer a new perspective on the structure of the inner disk, the stellar magnetosphere, and their interaction.
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