24 research outputs found
AA Tau's sudden and long-lasting deepening: enhanced extinction by its circumstellar disk
AA Tau has been monitored for more than 20 years since 1987, exhibiting a
nearly constant brightness level of V=12.5 mag. We report here that in 2011 it
suddenly faded away, becoming 2 magnitudes fainter in the V-band, and has
remained in this deep state since then. We report new optical and near-IR
photometry and spectroscopy obtained during the fading event. The system
appears much redder and fainter than it was in the bright state. Also, the 8.2d
photometric period continuously observed for more than 20 years is not seen
during most of the deep state. The analysis of the system's brightness and
colors suggests that the visual extinction on the line of sight has increased
by about 3-4 magnitudes in the deep state. At optical wavelengths, the system
appears to be dominated by scattered light, probably originating from the upper
surface layers of a highly inclined circumstellar disk. The profiles of the
Balmer lines have significantly changed as well, with the disappearance of a
central absorption component regularly observed in the bright state. We ascribe
this change to the scattering of the system's spectrum by circumstellar dust.
Noticeably, the mass accretion rate in the inner disk and onto the central star
has not changed as the system faded away. We conclude that the deepening of the
AA Tau system is due to a sudden increase of circumstellar dust extinction on
the line of sight without concomitant change in the accretion rate. We suggest
that the enhanced obscuration may be produced by a non-axisymmetric overdense
region in the disk, located at a distance of 7.7 AU or more, that was recently
brought on the line of sight by its keplerian motion around the central star.Comment: 9 pages, 6 figures, Astronomy & Astrophysics; english language
edited, email address update
Long-Term Photometric and Spectral Variations of DI Cephei
We have analyzed the photometric and spectral variations of the classical T Tauri star DI Cep
for the last 50 years. Currently the star is at its faintest state and possesses an emission spectrum
in the visual range. Synchronous spectroscopy and UBV R photometry show that the higher the
brightness, the stronger were the intensities of hydrogen Hα, Hβ emission lines and of FeII, HeI
λ5876 ˚A emissions. For the first time, we detected, with a high probability, quasi-periodic variations
of the star’s brightness and of its spectrum with the period P = 2020 ± 200 days
The 2008-2009 outburst of the young binary system Z CMa unraveled by interferometry with high spectral resolution
Z CMa is a young binary system consisting of an Herbig primary and a FU Ori
companion. Both components seem to be surrounded by active accretion disks and
a jet was associated to the Herbig B0. In Nov. 2008, K. Grankin discovered that
Z CMa was exhibiting an outburst with an amplitude larger than any photometric
variations recorded in the last 25 years. To study the innermost regions in
which the outburst occurs and understand its origin, we have observed both
binary components with AMBER/VLTI across the Br{\gamma} emission line in Dec.
2009 in medium and high spectral resolution modes. Our observations show that
the Herbig Be, responsible for the increase of luminosity, also produces a
strong Br{\gamma} emission, and they allow us to disentangle from various
origins by locating the emission at each velocities through the line.
Considering a model of a Keplerian disk alone fails at reproducing the
asymmetric spectro-astrometric measurements, suggesting a major contribution
from an outflow.Comment: To be published in the proceedings of the SPIE'2010 conference on
"Optical and Infrared Interferometry II
SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets Around Young Stars
We present a review of precursor observing programs for the SIM PlanetQuest
Key project devoted to detecting Jupiter mass planets around young stars. In
order to ensure that the stars in the sample are free of various sources of
astrometric noise that might impede the detection of planets, we have initiated
programs to collect photometry, high contrast images, interferometric data and
radial velocities for stars in both the Northern and Southern hemispheres. We
have completed a high contrast imaging survey of target stars in Taurus and the
Pleiades and found no definitive common proper motion companions within one
arcsecond (140 AU) of the SIM targets. Our radial velocity surveys have shown
that many of the target stars in Sco-Cen are fast rotators and a few stars in
Taurus and the Pleiades may have sub-stellar companions. Interferometric data
of a few stars in Taurus show no signs of stellar or sub-stellar companions
with separations of <5 mas. The photometric survey suggests that approximately
half of the stars initially selected for this program are variable to a degree
(1 sigma>0.1 mag) that would degrade the astrometric accuracy achievable for
that star. While the precursor programs are still a work in progress, we
provide a comprehensive list of all targets ranked according to their viability
as a result of the observations taken to date. By far, the observable that
moves the most targets from the SIM-YSO program is photometric variability.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific, 25 pages, 9 figure
The Architecture of the GW Ori Young Triple Star System and Its Disk: Dynamical Masses, Mutual Inclinations, and Recurrent Eclipses
We present spatially and spectrally resolved Atacama Large
Millimeter/submillimeter Array (ALMA) observations of gas and dust orbiting the
pre-main sequence hierarchical triple star system GW Ori. A forward-modeling of
the CO and CO =2-1 transitions permits a measurement of
the total stellar mass in this system, , and the
circum-triple disk inclination, . Optical spectra spanning
a 35 year period were used to derive new radial velocities and, coupled with a
spectroscopic disentangling technique, revealed that the A and B components of
GW Ori form a double-lined spectroscopic binary with a day
period; a tertiary companion orbits that inner pair with a day
period. Combining the results from the ALMA data and the optical spectra with
three epochs of astrometry in the literature, we constrain the individual
stellar masses in the system (,
, ) and
find strong evidence that at least one (and likely both) stellar orbital planes
are misaligned with the disk plane by as much as . A -band light
curve spanning 30 years reveals several new 30 day eclipse events
0.1-0.7~mag in depth and a 0.2 mag sinusoidal oscillation that is clearly
phased with the AB-C orbital period. Taken together, these features suggest
that the A-B pair may be partially obscured by material in the inner disk as
the pair approaches apoastron in the hierarchical orbit. Lastly, we conclude
that stellar evolutionary models are consistent with our measurements of the
masses and basic photospheric properties if the GW Ori system is 1 Myr
old.Comment: 26 pages, 15 figures, accepted to Ap
Placing the spotted T Tauri star LkCa 4 on an HR diagram
Ages and masses of young stars are often estimated by comparing their luminosities and effective temperatures to pre-main-sequence stellar evolution tracks, but magnetic fields and starspots complicate both the observations and evolution. To understand their influence, we study the heavily spotted weak-lined T-Tauri star LkCa 4 by searching for spectral signatures of radiation originating from the starspot or starspot groups. We introduce a new methodology for constraining both the starspot filling factor and the spot temperature by fitting two-temperature stellar atmosphere models constructed from Phoenix synthetic spectra to a high-resolution near-IR IGRINS spectrum. Clearly discernable spectral features arise from both a hot photospheric component Thot ∼ 4100 K and a cool component Tcool ∼ 2700–3000 K, which covers ∼80% of the visible surface. This mix of hot and cool emission is supported by analyses of the spectral energy distribution, rotational modulation of colors and of TiO band strengths, and features in low-resolution optical/near-IR spectroscopy. Although the revised effective temperature and luminosity make LkCa 4 appear to be much younger and of much lower mass than previous estimates from unspotted stellar evolution models, appropriate estimates will require the production and adoption of spotted evolutionary models. Biases from starspots likely afflict most fully convective young stars and contribute to uncertainties in ages and age spreads of open clusters. In some spectral regions, starspots act as a featureless "veiling" continuum owing to high rotational broadening and heavy line blanketing in cool star spectra. Some evidence is also found for an anticorrelation between the velocities of the warm and cool components.Peer reviewe
Possible Time Correlation between Jet Ejection and Mass Accretion for RW Aur A
For the active T-Taur star RW Aur A we have performed long-term (10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644 μm emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 A and O I 7772 A line profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past 15 yr with an irregular interval of 2-6 yr. The timescale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (ΔV < −1) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r lesssim 0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few astronomical units of the star, and therefore possible ongoing planet formation at these radii.M.T. is supported by the Ministry of Science and
Technology (MoST) of Taiwan (grant No. 106-2119-M-001-
026-MY3). R.G.M. acknowledges support from UNAMPAPIIT project IN104319. T.P.R. acknowledges support from
the European Research Council through grant No. 743029