3,573 research outputs found
A formal method for identifying distinct states of variability in time-varying sources: SgrA* as an example
Continuously time variable sources are often characterized by their power
spectral density and flux distribution. These quantities can undergo dramatic
changes over time if the underlying physical processes change. However, some
changes can be subtle and not distinguishable using standard statistical
approaches. Here, we report a methodology that aims to identify distinct but
similar states of time variability. We apply this method to the Galactic
supermassive black hole, where 2.2 um flux is observed from a source associated
with SgrA*, and where two distinct states have recently been suggested. Our
approach is taken from mathematical finance and works with conditional flux
density distributions that depend on the previous flux value. The discrete,
unobserved (hidden) state variable is modeled as a stochastic process and the
transition probabilities are inferred from the flux density time series. Using
the most comprehensive data set to date, in which all Keck and a majority of
the publicly available VLT data have been merged, we show that SgrA* is
sufficiently described by a single intrinsic state. However the observed flux
densities exhibit two states: a noise-dominated and a source-dominated one. Our
methodology reported here will prove extremely useful to assess the effects of
the putative gas cloud G2 that is on its way toward the black hole and might
create a new state of variability.Comment: Submitted to ApJ; 33 pages, 4 figures; comments welcom
Substellar multiplicity in the Hyades cluster
We present the first high-angular resolution survey for multiple systems
among very low-mass stars and brown dwarfs in the Hyades open cluster. Using
the Keck\,II adaptive optics system, we observed a complete sample of 16
objects with estimated masses 0.1 Msun. We have identified three
close binaries with projected separation 0.11", or 5 AU. A
number of wide, mostly faint candidate companions are also detected in our
images, most of which are revealed as unrelated background sources based on
astrometric and/or photometric considerations. The derived multiplicity
frequency, 19+13/-6 % over the 2-350 AU range, and the rarity of systems wider
than 10 AU are both consistent with observations of field very low-mass
objects. In the limited 3-50 AU separation range, the companion frequency is
essentially constant from brown dwarfs to solar-type stars in the Hyades
cluster, which is also in line with our current knowledge for field stars.
Combining the binaries discovered in this surveys with those already known in
the Pleiades cluster reveals that very low-mass binaries in open clusters, as
well as in star-forming regions, are skewed toward lower mass ratios () than are their field counterparts, a result that
cannot be accounted for by selection effects. Although the possibility of
severe systematic errors in model-based mass estimates for very low-mass stars
cannot be completely excluded, it is unlikely to explain this difference. We
speculate that this trend indicates that surveys among very low-mass field
stars may have missed a substantial population of intermediate mass ratio
systems, implying that these systems are more common and more diverse than
previously thought.Comment: Accepted for publication in Astronomy & Astrophysics; 11 pages, 6
figure
A Two-Dimensional Near-Infrared Speckle Imaging Survey of T Tauri Stars in Taurus and Ophiuchus
We present the results of a magnitude limited (K≤8.5 mag) multiplicity survey of T Tauri stars in the two nearest star forming regions, Taurus-Auriga and Ophiuchus-Scorpius (D = 150 pc), observable from the northern hemisphere. Each of the 70 stars in the sample was imaged at 2.2 μm using two-dimensional speckle interferometry resulting in a survey sensitive to binary stars with separations ranging from 0.″09 to about 2″.5.
The frequency of double stars with separation in this range is 46±8%. A division between the classical T Tauri stars and the weak-lined T Tauri stars shows no distinction. Furthermore, no difference is observed between the binary frequencies in the two star forming regions although the clouds have very different properties.
Given the limited angular separation range that this survey is sensitive to, both the spectroscopic and wide binaries will be missed. The rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
The frequency of T Tauri companion stars
We present the results of a magnitude limited (K≤8.5 mag) multiplicity survey of T Tauri stars in two nearby star forming regions, Tauris-Auriga and Ophiuchus-Scorpius. Each of the 69 stars in the sample was observed at K(2.2 microns) with an infrared array camera on the Hale 5-m Telescope at Palomar Observatory and imaged using two-dimensional speckle imaging techniques. Thirty three companion stars were found of which 15 were new detections. One of the main results of this survey indicates that the binary star frequncy in the projected linear separation range 14 to 225 AU for T Tauri stars (59 ± 16%) is a factor of 3.5 greater than that of the solar-type main sequence stars (17 ±3%). Given the limited angular separation range of this survey, i.e., both the spectroscopic and wide binaries are missed, the rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions. We propose that the observed overabundance of companions to the T Tauri stars relative to their older counterparts on the main sequence is an evolutionary effect; in this scheme triple and higher order T Tauri star systems, which are observed at higher frequencies than for the solar-type main sequence stars, are disrupted by close encounters with another star or system of stars
The multiplicity of T Tauri stars in the star forming regions Taurus-Auriga and Ophiuchus-Scorpius: A 2.2 micron speckle imaging survey
We present the results of a magnitude limited (K < =8.5 mag) speckle imaging survey of 69 T Tauri stars in the star forming regions Taurus-Auriga and Ophiuchus-Scorpius. Thirty-three companion stars were found with separations ranging from 0."07 to 2."5; nine are new detections. This survey reveals a distinction between the classical T Tauri stars (CTTS) and the weak-lined T Tauri stars (WTTS) based on the binary star frequency as a function of separation: the WTTS binary star distribution is enhanced at the closer separations (<50 AU) relative to the CTTS binary star distribution. We suggest that the nearby companion stars shorten the accretion time scale in multiple star systems, thereby accounting for the presence of WTTS that are coeval with many CTTS. The binary star frequency in the projected linear separation range 16 to 252 AU for T Tauri stars (60[± 17]%) is a factor of 4 greater than that of the solar-type main-sequence stars (16[±3]%). Given the limited separation range of this survey, the rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions. We propose that the observed overabundance of companions of T Tauri stars is an evolutionary effect, in which triple and higher order T Tauri stars are disrupted by close encounters with another star or system of stars
First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution
Investigating the environment of the massive black hole SgrA* at the center
of the Galaxy requires the highest angular resolution available to avoid source
confusion and to study the physical properties of the individual objects.
GCIRS7 has been used as wavefront and astrometric reference. Our studies
investigate, for the first time, its properties at 2&10um using VLTI/AMBER and
MIDI. We aim at analyzing the suitability of IRS7 as an IF-phase-reference for
the upcoming generation of dual-field facilities at optical interferometers. We
observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas
resolution for NIR and MIR, resp. The first K-band fringe detection of a GC
star suggests that IRS7 could be marginally resolved at 2um, which would imply
that the photosphere of the supergiant is enshrouded by a molecular and dusty
envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately
0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly
distributed outside of a radius of about 120 AU (15 mas) around the star. A
deep 9.8-silicate absorption in excess of the usual extinction law with respect
to the NIR extinction has been found. This confirms recent findings of a
relatively enhanced, interstellar 9.8-silicate absorption with respect to the
NIR extinction towards another star in the central arcsec, suggesting an
unusual dust composition in that region. Our VLTI observations show that
interferometric NIR phase-referencing experiments with mas resolution using
IRS7 as phase-reference appear to be feasible, but more such studies are
required to definitely characterize the close environment around this star. We
demonstrate that interferometry is required to resolve the innermost
environment of stars at the Galactic center.Comment: 6 pages, 2 figures, accepted for publication in A&
NICMOS Images of the GG Tau Circumbinary Disk
We present deep, near-infrared images of the circumbinary disk surrounding
the pre-main-sequence binary star, GG Tau A, obtained with NICMOS aboard the
Hubble Space Telescope. The spatially resolved proto-planetary disk scatters
roughly 1.5% of the stellar flux, with a near-to-far side flux ratio of ~1.4,
independent of wavelength, and colors that are comparable to the central
source; all of these properties are significantly different from the earlier
ground-based observations. New Monte Carlo scattering simulations of the disk
emphasize that the general properties of the disk, such as disk flux, near side
to far side flux ratio and integrated colors, can be approximately reproduced
using ISM-like dust grains, without the presence of either circumstellar disks
or large dust grains, as had previously been suggested. A single parameter
phase function is fitted to the observed azimuthal variation in disk flux,
providing a lower limit on the median grain size of 0.23 micron. Our analysis,
in comparison to previous simulations, shows that the major limitation to the
study of grain growth in T Tauri disk systems through scattered light lies in
the uncertain ISM dust grain properties. Finally, we use the 9 year baseline of
astrometric measurements of the binary to solve the complete orbit, assuming
that the binary is coplanar with the circumbinary ring. We find that the
estimated 1 sigma range on disk inner edge to semi-major axis ratio, 3.2 <
Rin/a < 6.7, is larger than that estimated by previous SPH simulations of
binary-disk interactions.Comment: 40 pages, 8 postscript figures, accepted for publication in Ap
- …