2,319 research outputs found
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
The ALMA Early Science View of FUor/EXor objects. III. The Slow and Wide Outflow of V883 Ori
We present Atacama Large Millimeter/ sub-millimeter Array (ALMA) observations
of V883 Ori, an FU Ori object. We describe the molecular outflow and envelope
of the system based on the CO and CO emissions, which together
trace a bipolar molecular outflow. The CO emission traces the rotational
motion of the circumstellar disk. From the CO blue-shifted emission, we
estimate a wide opening angle of 150 for the outflow
cavities. Also, we find that the outflow is very slow (characteristic velocity
of only 0.65 km~s), which is unique for an FU Ori object. We calculate
the kinematic properties of the outflow in the standard manner using the
CO and CO emissions. In addition, we present a P Cygni profile
observed in the high-resolution optical spectrum, evidence of a wind driven by
the accretion and being the cause for the particular morphology of the
outflows. We discuss the implications of our findings and the rise of these
slow outflows during and/or after the formation of a rotationally supported
disk.Comment: 12 pages, 7 figures, 2 tables. Accepte
A Chemical Map of the Outbursting V883 Ori system: Vertical and Radial Structures
We present the first results of a pilot program to conduct an Atacama Large
Millimeter/submillimeter Array (ALMA) Band 6 (211-275 GHz) spectral line study
of young stellar objects (YSO) that are undergoing rapid accretion episodes,
i.e. FU Ori objects (FUors). Here, we report on molecular emission line
observations of the FUor system, V883 Ori. In order to image the FUor object
with full coverage from ~0.5 arcsec to the map size of ~30 arcsec, i.e. from
disc to outflow scales, we combine the ALMA main array (the 12-m array) with
the Atacama Compact Array (7-m array) and the total power (TP) array. We detect
HCN, HCO, CHOH, SO, DCN, and HCO emissions with most of these
lines displaying complex kinematics. From PV diagrams, the detected molecules
HCN, HCO, CHOH, DCN, SO, and HCO probe a Keplerian rotating
disc in a direction perpendicular to the large-scale outflow detected
previously with the CO and CO lines. Additionally, HCN and
HCO reveal kinematic signatures of infall motion. The north outflow is
seen in HCO, HCO, and SO emissions. Interestingly, HCO
emission reveals a pronounced inner depression or "hole" with a size comparable
to the radial extension estimated for the CHOH and 230 GHz continuum. The
inner depression in the integrated HCO intensity distribution of V883 Ori
is most likely the result of optical depth effects, wherein the optically thick
nature of the HCO and continuum emission towards the innermost parts of
V883 Ori can result in a continuum subtraction artifact in the final HCO
flux level
Optical Spectral Variability of the Very-High-Energy Gamma-Ray Blazar 1ES 1011+496
We present results of five years of optical (UBVRI) observations of the
very-high-energy gamma-ray blazar 1ES 1011+496 at the MDM Observatory. We
calibrated UBVRI magnitudes of five comparison stars in the field of the
object. Most of our observations were done during moderately faint states of
1ES 1011+496 with R > 15.0. The light curves exhibit moderate, closely
correlated variability in all optical wavebands on time scales of a few days. A
cross-correlation analysis between optical bands does not show significant
evidence for time lags. We find a positive correlation (Pearson's r = 0.57;
probability of non-correlation P(>r) ~ 4e-8) between the R-band magnitude and
the B - R color index, indicating a bluer-when-brighter trend. Snap-shot
optical spectral energy distributions (SEDs) exhibit a peak within the optical
regime, typically between the V and B bands. We find a strong (r = 0.78;
probability of non-correlation P (>r) ~ 1e-15) positive correlation between the
peak flux and the peak frequency, best fit by a relation with k = 2.05 +/- 0.17. Such a correlation is
consistent with the optical (synchrotron) variability of 1ES 1011+496 being
primarily driven by changes in the magnetic field.Comment: Accepted for publication in ApJ. 16 pages, including 7 figure
Human hippocampal theta oscillations reflect sequential dependencies during spatial planning
Movement-related theta oscillations in rodent hippocampus coordinate ‘forward sweeps’ of location-specific neural activity that could be used to evaluate spatial trajectories online. This raises the possibility that increases in human hippocampal theta power accompany the evaluation of upcoming spatial choices. To test this hypothesis, we measured neural oscillations during a spatial planning task that closely resembles a perceptual decision-making paradigm. In this task, participants searched visually for the shortest path between a start and goal location in novel mazes that contained multiple choice points, and were subsequently asked to make a spatial decision at one of those choice points. We observed ~4–8 Hz hippocampal/medial temporal lobe theta power increases specific to sequential planning that were negatively correlated with subsequent decision speed, where decision speed was inversely correlated with choice accuracy. These results implicate the hippocampal theta rhythm in decision tree search during planning in novel environments
Hubble Space Telescope Observations of an Outer Field in Omega Centauri: A Definitive Helium Abundance
We revisit the problem of the split main sequence (MS) of the globular
cluster omega Centauri, and report the results of two-epoch Hubble Space
Telescope observations of an outer field, for which proper motions give us a
pure sample of cluster members, and an improved separation of the two branches
of the main sequence. Using a new set of stellar models covering a grid of
values of helium and metallicity, we find that the best possible estimate of
the helium abundance of the bluer branch of the MS is Y = 0.39 +/- 0.02. For
the cluster center we apply new techniques to old observations: we use indices
of photometric quality to select a high-quality sample of stars, which we also
correct for differential reddening. We then superpose the color-magnitude
diagram of the outer field on that of the cluster center, and suggest a
connection of the bluer branch of the MS with one of the more prominent among
the many sequences in the subgiant region. We also report a group of undoubted
cluster members that are well to the red of the lower MS.Comment: 26 pages, 10 figures (4 in low resolution. AJ accepted on March 21,
201
Alma Survey Of Circumstellar Disks In The Young Stellar Cluster IC 348
We present a 1.3 mm continuum survey of the young (2-3 Myr) stellar cluster
IC 348, which lies at a distance of 310 pc, and is dominated by low-mass stars
(M 0.1-0.6 M). We observed 136 Class II sources
(disks that are optically thick in the infrared) at 0.8 (200 au) resolution
with a 3 sensitivity of 0.45 mJy (M 1.3
M). We detect 40 of the targets and construct a mm-continuum
luminosity function. We compare the disk mass distribution in IC 348 to those
of younger and older regions, taking into account the dependence on stellar
mass. We find a clear evolution in disk masses from 1 to 5-10 Myr. The disk
masses in IC 348 are significantly lower than those in Taurus (1-3 Myr) and
Lupus (1-3 Myr), similar to those of Chamaleon~I, (2-3 Myr) and Ori
(3-5 Myr) and significantly higher than in Upper Scorpius (510 Myr). About
20 disks in our sample (5 of the cluster members) have estimated
masses (dust gas) 1 M and hence might be the precursors of
giant planets in the cluster. Some of the most massive disks include transition
objects with inner opacity holes based on their infrared SEDs. From a stacking
analysis of the 96 non-detections, we find that these disks have a typical dust
mass of just 0.4 M, even though the vast majority of
their infrared SEDs remain optically thick and show little signs of evolution.
Such low-mass disks may be the precursors of the small rocky planets found by
\emph{Kepler} around M-type stars.Comment: 19 pages, 12 figure
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