2,520 research outputs found
Evidence for Variable, Correlated X-ray and Optical/IR Extinction toward the Nearby, Pre-main Sequence Binary TWA 30
We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR
spectroscopic observations of the nearby ( pc), low-mass (mid-M)
binary system TWA 30A and 30B. The components of this wide (separation
3400 AU) binary are notable for their nearly edge-on disk viewing
geometries, high levels of variability, and evidence for collimated stellar
outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011
June and July, accompanied (respectively) by IRTF SpeX (near-IR) and VLT
XSHOOTER (visible/near-IR) spectroscopy obtained within 20 hours of the
X-ray observations. TWA 30A was detected in both XMM-Newton observations at
relatively faint intrinsic X-ray luminosities (
) compared to stars of similar mass and age . The intrinsic
(0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor
20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected,
and we infer an upper limit of ( 3.0 erg
s). We measured a large change in visual extinction toward TWA 30A (from
to ) between the two 2011 observing epochs,
and we find evidence for a corresponding significant decrease in X-ray
absorbing column (). The apparent correlated change in and is
suggestive of variable obscuration of the stellar photosphere by disk material
composed of both gas and dust. However, in both observations, the inferred
to ratio is lower than that typical of the ISM, suggesting that
the disk is either depleted of gas or is deficient in metals in the gas phase.Comment: 10 pages, 7 figures, Accepted for publication in MNRA
The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity
We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of
HBC 515, a system consisting of multiple young stellar objects (YSOs). The five
members of HBC 515 represent a remarkably diverse array of YSOs, ranging from
the low-mass Class I/II protostar HBC 515B, through Class II and transition
disk objects (HBC 515D and C, respectively), to the "diskless", intermediate-
mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes
that all five components are X-ray sources, with HBC 515A - a
subarcsecond-separation binary that is partially resolved by Chandra - being
the dominant X-ray source. We detect an X-ray flare associated with HBC 515B.
In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission
source; a second, weaker mm continuum source is coincident with the position of
the transition disk object HBC 515C. These results strongly support the
protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of
the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars"
that are binaries and have shed their disks at very early stages of pre-MS
evolution. The coexistence of two such disparate objects within a single,
presumably coeval multiple YSO system highlights the influence of pre- MS star
mass, binarity, and X-ray luminosity in regulating the lifetimes of
circumstellar, planet-forming disks and the timescales of star-disk
interactions.Comment: Accepted for publication in A&A; 11 pages, 5 figure
Maximum Entropy Linear Manifold for Learning Discriminative Low-dimensional Representation
Representation learning is currently a very hot topic in modern machine
learning, mostly due to the great success of the deep learning methods. In
particular low-dimensional representation which discriminates classes can not
only enhance the classification procedure, but also make it faster, while
contrary to the high-dimensional embeddings can be efficiently used for visual
based exploratory data analysis.
In this paper we propose Maximum Entropy Linear Manifold (MELM), a
multidimensional generalization of Multithreshold Entropy Linear Classifier
model which is able to find a low-dimensional linear data projection maximizing
discriminativeness of projected classes. As a result we obtain a linear
embedding which can be used for classification, class aware dimensionality
reduction and data visualization. MELM provides highly discriminative 2D
projections of the data which can be used as a method for constructing robust
classifiers.
We provide both empirical evaluation as well as some interesting theoretical
properties of our objective function such us scale and affine transformation
invariance, connections with PCA and bounding of the expected balanced accuracy
error.Comment: submitted to ECMLPKDD 201
Molecules in the transition disk orbiting T Cha
We seek to establish the presence and properties of gas in the circumstellar
disk orbiting T Cha, a nearby (d~110 pc), relatively evolved (age ~5-7 Myr) yet
actively accreting 1.5 Msun T Tauri star. We used the APEX 12 m radiotelescope
to search for submillimeter molecular emission from the T Cha disk, and we
reanalyzed archival XMM-Newton spectroscopy of T Cha to ascertain the
intervening absorption due to disk gas along the line of sight to the star
(N_H). We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN
and HCO+ from the T Cha disk. The 12CO line appears to display a double-peaked
line profile indicative of Keplerian rotation. Analysis of the CO emission line
data indicates that the disk around T Cha has a mass (M_disk,H_2 = 80 M_earth)
similar to, but more compact (R_disk, CO~80 AU) than, other nearby, evolved
molecular disks (e.g. V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas
has been previously detected. The HCO+/13CO and HCN/13CO, line ratios measured
for T Cha appear similar to those of other evolved circumstellar disks (i.e. TW
Hya and V4046 Sgr), while the CN/13CO ratio appears somewhat weaker. Analysis
of the XMM-Newton data shows that the atomic absorption toward T Cha is
1-2 orders of magnitude larger than toward the other nearby T Tauri with
evolved disks. Furthermore, the ratio between atomic absorption and optical
extinction N_H/A_V toward T Cha is higher than the typical value observed for
the interstellar medium and young stellar objects in the Orion Nebula Cluster.
This may suggest that the fraction of metals in the disk gas is higher than in
the interstellar medium. Our results confirm that pre-main sequence stars older
than ~5 Myr, when accreting, retain cold molecular disks, and that those
relatively evolved disks display similar physical and chemical properties.Comment: Accepted for publication on A&
A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars
We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the
globular cluster Omega Cen (NGC 5139) which we combine with data from the
literature to construct a sample of 180 RR Lyrae stars with J and K mean
magnitudes on a common photometric system. This is presently the largest such
sample in any stellar system. We also present updated predictions for J,K-band
Period-Luminosity relations for both fundamental and first-overtone RR Lyrae
stars, based on synthetic horizontal branch models with metal abundance ranging
from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured
metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex)
with respect to iron we find a true distance modulus of 13.70 (with a random
error of 0.06 and a systematic error of 0.06), corresponding to a distance
d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our
estimate is in excellent agreement with the distance inferred for the eclipsing
binary OGLEGC-17, but differ significantly from the recent distance estimates
based on cluster dynamics and on high amplitude Delta Scuti stars.Comment: 24 pages, 7 figures, accepted for publication on The Astrophysical
Journa
Evaluation of fiber-optic phase-gradient meta-tips for sensing applications
Recently, within the emerging framework of "lab-on-fiber" technologies, we successfully demonstrated the integration of phase-gradient plasmonic metasurfaces on the tip of an optical fiber. The res..
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