457 research outputs found
Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264
We present some results from a UVI photometric study of a field in the young
open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main
sequence stars is linked to rotation. We confirm that U-V excess is well
correlated with H-alpha equivalent width for the stars in our sample. We show
that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun)
there is also a significant association between U-V excess and rotation, in the
sense that slow rotators are more likely to show excess U-band emission and
variability. This constitutes significant new evidence in support of the
disk-locking paradigm.Comment: Accepted by ApJ Letter
X-ray Emission from the Weak-lined T Tauri Binary System KH 15D
The unique eclipsing, weak-lined T Tauri star KH 15D has been detected as an
X-ray source in a 95.7 ks exposure from the Chandra X-ray Observatory archives.
A maximum X-ray luminosity of 1.5 x 10^{29} erg s is derived in the
0.5--8 keV band, corresponding to L_{X}/L_bol = 7.5 x 10^{-5}. Comparison with
samples of stars of similar effective temperature in NGC 2264 and in the Orion
Nebula Cluster shows that this is about an order of magnitude low for a typical
star of its mass and age. We argue that the relatively low luminosity cannot be
attributed to absorption along the line of sight but implies a real deficiency
in X-ray production. Possible causes for this are considered in the context of
a recently proposed eccentric binary model for KH 15D. In particular, we note
that the visible component rotates rather slowly for a weak-lined T Tauri star
and has possibly been pseudosynchronized by tidal interaction with the primary
near periastron
Bipolar jets produced by a spectroscopic binary
We present evidence that the spectroscopically identified bipolar jets of the
pre-main sequence binary KH 15D are a common product of the whole binary
system, rather than being launched from either star individually. They may be
launched from the innermost part of the circumbinary disk (CBD) or may result
from the merging of two outflows driven by the individual stars. This evidence
is based on high-resolution H-alpha and [OI] 6300A line profiles obtained
during eclipse phases of this nearly edge-on system. The occultation of star A
(the only currently visible star) by the disk strongly suppresses the stellar
H-alpha and continuum emission and allows one to study the faint redshifted and
blueshifted emission components of the bipolar jets. The strongest evidence for
jet production by the whole binary system comes from the observed radial
velocity symmetry of the two jet components relative to the systemic velocity
of the binary, in combination with current accretion models from the CBD onto a
binary system.Comment: ApJ Letters, in press [6 pages
Natural Coronagraphic Observations of the Eclipsing T Tauri System KH 15D: Evidence for Accretion and Bipolar Outflow in a WTTS
We present high resolution (R 44,000) UVES spectra of the eclipsing
pre-main sequence star KH 15D covering the wavelength range 4780 to 6810 {\AA}
obtained at three phases: out of eclipse, near minimum light and during egress.
The system evidently acts like a natural coronagraph, enhancing the contrast
relative to the continuum of hydrogen and forbidden emission lines during
eclipse. At maximum light the H equivalent width was 2 {\AA} and
the profile showed broad wings and a deep central absorption. During egress the
equivalent width was much higher (70 {\AA}) and the broad wings, which
extend to 300 km/s, were prominent. During eclipse totality the
equivalent width was less than during egress (40 {\AA}) and the high
velocity wings were much weaker. H showed a somewhat different behavior,
revealing only the blue-shifted portion of the high velocity component during
eclipse and egress. [OI] 6300, 6363 lines are easily seen both
out of eclipse and when the photosphere is obscured and exhibit little or no
flux variation with eclipse phase. Our interpretation is that KH 15D, although
clearly a weak-line T Tauri star by the usual criteria, is still accreting
matter from a circumstellar disk, and has a well-collimated bipolar jet. As the
knife-edge of the occulting matter passes across the close stellar environment
it is evidently revealing structure in the magnetosphere of this pre-main
sequence star with unprecedented spatial resolution. We also show that there is
only a small, perhaps marginally significant, change in the velocity of the K7
star between the maximum light and egress phases probed here
The Highly Dynamic Behavior of the Innermost Dust and Gas in the Transition Disk Variable LRLL 31
We describe extensive synoptic multi-wavelength observations of the
transition disk LRLL 31 in the young cluster IC 348. We combined four epochs of
IRS spectra, nine epochs of MIPS photometry, seven epochs of cold-mission IRAC
photometry and 36 epochs of warm mission IRAC photometry along with multi-epoch
near-infrared spectra, optical spectra and polarimetry to explore the nature of
the rapid variability of this object. We find that the inner disk, as traced by
the 2-5micron excess stays at the dust sublimation radius while the strength of
the excess changes by a factor of 8 on weekly timescales, and the 3.6 and
4.5micron photometry shows a drop of 0.35 magnitudes in one week followed by a
slow 0.5 magnitude increase over the next three weeks. The accretion rate, as
measured by PaBeta and BrGamma emission lines, varies by a factor of five with
evidence for a correlation between the accretion rate and the infrared excess.
While the gas and dust in the inner disk are fluctuating the central star stays
relatively static. Our observations allow us to put constraints on the physical
mechanism responsible for the variability. The variabile accretion, and wind,
are unlikely to be causes of the variability, but both are effects of the same
physical process that disturbs the disk. The lack of periodicity in our
infrared monitoring indicates that it is unlikely that there is a companion
within ~0.4 AU that is perturbing the disk. The most likely explanation is
either a companion beyond ~0.4 AU or a dynamic interface between the stellar
magnetic field and the disk leading to a variable scale height and/or warping
of the inner disk.Comment: Accepted to ApJ. 10 pages of text, plus 11 tables and 13 figures at
the en
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