3,860 research outputs found
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
MHD simulations of disk-star interaction
We discuss a number of topics relevant to disk-magnetosphere interaction and
how numerical simulations illuminate them. The topics include: (1)
disk-magnetosphere interaction and the problem of disk-locking; (2) the wind
problem; (3) structure of the magnetospheric flow, hot spots at the star's
surface, and the inner disk region; (4) modeling of spectra from 3D funnel
streams; (5) accretion to a star with a complex magnetic field; (6) accretion
through 3D instabilities; (7) magnetospheric gap and survival of protoplanets.
Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars,
Proceedings of the International Astronomical Union, IAU Symposium, Volume
243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at
http://astro.cornell.edu/us-rus
A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods
We use mid-IR (primarily 10 m) photometry as a diagnostic for the
presence of disks with inner cavities among 32 pre-main sequence stars in Orion
and Taurus-Auriga for which rotation periods are known and which do not show
evidence for inner disks at near-IR wavelengths. Disks with inner cavities are
predicted by magnetic disk-locking models that seek to explain the regulation
of angular momentum in T Tauri stars. Only three stars in our sample show
evidence for excess mid-IR emission. While these three stars may possess
truncated disks consistent with magnetic disk-locking models, the remaining 29
stars in our sample do not. Apparently, stars lacking near-IR excesses in
general do not possess truncated disks to which they are magnetically coupled.
We discuss the implications of this result for the hypothesis of
disk-regulated angular momentum. Evidently, young stars can exist as slow
rotators without the aid of present disk-locking, and there exist very young
stars already rotating near breakup velocity whose subsequent angular momentum
evolution will not be regulated by disks. Moreover, we question whether disks,
when present, truncate in the manner required by disk-locking scenarios.
Finally, we discuss the need for rotational evolution models to take full
account of the large dispersion of rotation rates present at 1 Myr, which may
allow the models to explain the rotational evolution of low-mass pre-main
sequence stars in a way that does not depend upon braking by disks.Comment: 20 pages, 4 figure
A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope
We present results showing that the multiple system DENIS-P J020529.0-115925
is likely to be a triple system of brown dwarfs. The secondary of this
previously known binary system shows a clear elongation on six images obtained
at six different epochs. Significant residuals remain after PSF subtraction on
these images, characteristic of multiplicity, and indicating that the secondary
is probably a double itself. Dual-PSF fitting shows that the shape of the
secondary is consistent with that of a binary system. These measurements show
that the probability that DENIS-P J020529.0-115925 is a triple system is very
high. The photometric and spectroscopic properties of the system are consistent
with the presence of three components with spectral types L5, L8 and T0.Comment: 15 pages, 3 tables, 6 figures, accepted for publication in AJ. High
resolution version available at
ftp://ftp.mpe.mpg.de/people/hbouy/publications/denis0205.ps.g
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: I. Magnetospheric Flow at Different Theta
We present results of fully three-dimensional MHD simulations of disk
accretion to a rotating magnetized star with its dipole moment inclined at an
angle Theta to the rotation axis of the disk. We observed that matter accretes
from the disk to a star in two or several streams depending on Theta. Streams
may precess around the star at small Theta. The inner regions of the disk are
warped. The warping is due to the tendency of matter to co-rotate with inclined
magnetosphere. The accreting matter brings positive angular momentum to the
(slowly rotating) star tending to spin it up. The corresponding torque N_z
depends only weakly on Theta. The angular momentum flux to the star is
transported predominantly by the magnetic field; the matter component
contributes < 1 % of the total flux. Results of simulations are important for
understanding the nature of classical T Tauri stars, cataclysmic variables, and
X-ray pulsars.Comment: 26 pages, 22 figures, LaTeX, macros: emulapj.sty, avi simulations are
available at http://www.astro.cornell.edu/us-rus/inclined.ht
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
Keck NIRC Observations of Planetary-mass Candidate Members in the sigma Orionis Open Cluster
We present K-band photometry and low-resolution near-infrared spectroscopy
from 1.44 to 2.45 micron of isolated planetary-mass candidate members in the
sigma Orionis cluster found by Zapatero Osorio et al. The new data have been
obtained with NIRC at the Keck I telescope. All of our targets, except for one,
are confirmed as likely cluster members. Hence, we also confirm that the
planetary-mass domain in the cluster is well populated. Using our deep K-band
images we searched for companions to the targets in the separation range 0.3"
to 10" up to a maximum faint limit of K=19.5 mag. One suspected companion seems
to be an extremely red galaxy. The near-infrared colors of the sigma Orionis
substellar members indicate that dust grains condense and settle in their
atmospheres. We estimate that the surface temperatures range from 2500 K down
to 1500 K. The spectroscopic sequence covers the full range of L subclasses,
and the faintest object is tentatively classified as T0. These targets provide
a sequence of substellar objects of known age, distance and metallicity, which
can be used as benchmark for understanding the spectral properties of ultracool
dwarfs.Comment: Accepted for publication in ApJ Letters. 9 pages, 4 figures include
Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core
We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100
stars within and surrounding the cloud core of rho Oph. We have measured
spectral types and continuum veilings and have combined this information with
results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly
declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic
units (Salpeter is +1.35). Our lower limits on the numbers of substellar
objects demonstrate that the IMF probably does not fall more steeply below the
hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first
comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, &
Neri) measured in the same region. The similar behavior of the two mass
functions in rho Oph supports the suggestion of Motte et al. and Testi &
Sargent that the stellar mass function in young clusters is a direct product of
the process of cloud fragmentation. After considering the effect of extinction
on the SED classifications of the sample, we find that ~17% of the rho Oph
stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra
separated by two years, we observe simultaneous variability in the Br gamma
emission and K-band continuum veiling for two stars, where the hydrogen
emission is brighter in the more heavily veiled data. This behavior indicates
that the disk may contribute significantly to continuous K-band emission, in
contrast to the proposal that the infalling envelope always dominates. Our
detection of strong 2 micron veiling (r_K=1-4) in several Class II and III
stars, which should have disks but little envelope material, further supports
this proposition.Comment: 35 pages, 14 figures, accepted to Ap
Spectropolarimetry and Modeling of the Eclipsing T Tauri Star KH 15D
KH 15D is a strongly variable T Tauri star in the young star cluster NGC 2264
that shows a decrease in flux of 3.5 magnitudes lasting for 18 days and
repeating every 48 days. The eclipsing material is likely due to orbiting dust
or rocky bodies in a partial ring or warped disk that periodically occults the
star. We measured the polarized spectrum in and out of eclipse at the Keck and
Palomar observatories. Outside of the eclipse, the star exhibited low
polarization consistent with zero. During eclipse, the polarization increased
dramatically to ~2% across the optical spectrum, while the spectrum had the
same continuum shape as outside of eclipse and exhibited emission lines of much
larger equivalent width, as previously seen. From the data, we conclude that
(a) the scattering region is uneclipsed; (b) the scattering is nearly
achromatic; (c) the star is likely completely eclipsed so that the flux during
eclipse is entirely due to scattered light, a conclusion also argued for by the
shape of the ingress and egress. We argue that the scattering is not due to
electrons, but may be due to large dust grains of size ~10 micron, similar to
the interplanetary grains which scatter the zodiacal light. We construct a
warped-disk model with an extended dusty atmosphere which reproduces the main
features of the lightcurve, namely (a) a gradual decrease before ingress due to
extinction in the atmosphere (similar for egress); (b) a sharper decrease
within ingress due to the optically-thick base of the atmosphere; (c) a
polarized flux during eclipse which is 0.1% of the total flux outside of
eclipse, which requires no fine-tuning of the model. (abridged)Comment: 9 pages, 7 figures, accepted for publication in ApJ, MPEG simulation
available at http://www.astro.washington.edu/agol/scatter2.mp
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