128 research outputs found
Long-lived protoplanetary disks in multiple systems: the VLA view of HD 98800
The conditions and evolution of protoplanetary disks in multiple systems can
be considerably different from those around single stars, which may have
important consequences for planet formation. We present Very Large Array (VLA)
8.8 mm (34 GHz) and 5 cm (6 GHz) observations of the quadruple system HD 98800,
which consists of two spectroscopic binary systems (Aa-Ab, Ba-Bb). The Ba-Bb
pair is surrounded by a circumbinary disk, usually assumed to be a debris disk
given its 10 Myr age and lack of near infrared excess. The VLA 8.8 mm
observations resolve the disk size (5-5.5 au) and its inner cavity (3
au) for the first time, making it one of the smallest disks known. Its small
size, large fractional luminosity, and millimeter spectral index consistent
with blackbody emission support the idea that HD 98800 B is a massive,
optically thick ring which may still retain significant amounts of gas. The
disk detection at 5 cm is compatible with free-free emission from photoionized
material. The diskless HD 98800 A component is also detected, showing partial
polarization at 5 cm compatible with non-thermal chromospheric activity. We
propose that tidal torques from Ba-Bb and A-B have stopped the viscous
evolution of the inner and outer disk radii, and the disk is evolving via mass
loss through photoevaporative winds. This scenario can explain the properties
and longevity of HD 98800 B as well as the lack of a disk around HD 98800 A,
suggesting that planet formation could have more time to proceed in multiple
systems than around single stars in certain system configurations.Comment: 14 pages, 4 figures, 3 tables; Submitted to ApJ May 14 2018; Accepted
to ApJ August 3 2018. This version fixes a mistake in the reported position
angle. The order of the figures has been changed to match that of the
references in the tex
A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628
Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of
black hole masses and accretion disks, and here we apply these diagnostics to
an ultraluminous X-ray source (ULX) in the spiral galaxy NGC628 (M74). This
object was observed four times over two years with the Chandra X-ray
Observatory and XMM-Newton, with three long observations showing dramatic
variability, distinguished by a series of outbursts with a quasi-period (QPO)
of 4,000-7,000 seconds. This is unique behavior among both ULXs and Galactic
X-ray binaries due to the combination of its burst-like peaks and deep troughs,
its long quasi-periods, its high variation amplitudes of %, and its
substantial variability between observations. The X-ray spectra is fitted by an
absorbed accretion disk plus a power-law component, suggesting the ULX was in a
spectral state analogous to the Low Hard state or the Very High state of
Galactic black hole X-ray binaries. A black hole mass of -- is estimated from the -- scaling relation found in the
Galactic X-ray binaries and active galactic nuclei.Comment: 12 pages, 3 figures. accepted for publication in ApJ Lette
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