277 research outputs found
Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release
The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of
2000 UV-selected objects that are candidate nearby (150 pc), young (age 10--100 Myr), late-type stars.
Here, we evaluate the distances and ages of the subsample of (19) GALNYSS stars
with Gaia Data Release 1 (DR1) parallax distances pc. The overall
youth of these 19 mid-K to early-M stars is readily apparent from their
positions relative to the loci of main sequence stars and giants in Gaia-based
color-magnitude and color-color diagrams constructed for all Galex- and
WISE-detected stars with parallax measurements included in DR1. The isochronal
ages of all 19 stars lie in the range 10--100 Myr. Comparison with
Li-based age estimates indicates a handful of these stars may be young
main-sequence binaries rather than pre-main sequence stars. Nine of the 19
objects have not previously been considered as nearby, young stars, and all but
one of these are found at declinations north of 30. The Gaia DR1
results presented here indicate that the GALNYSS sample includes several
hundred nearby, young stars, a substantial fraction of which have not been
previously recognized as having ages 100 Myr.Comment: 30 pages, 4 tables, 7 figures; to appear in The Astrophysical
Journal; 1st replacement to correct typos/omissions in Table 3 and
acknowledgments; 2nd replacement to incorporate corrections to ApJ proof
Evidence for Accretion in the High-resolution X-ray Spectrum of the T Tauri Star System Hen 3-600
We present high-resolution X-ray spectra of the multiple T Tauri star system
Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph on
the Chandra X-ray Observatory. Two binary components were detected in the
zeroth-order image. Hen 3-600-A, which has a large mid-infrared excess, is a
2-3 times fainter in X-rays than Hen 3-600-B, due to a large flare on B. The
dispersed X-ray spectra of the two primary components overlap spatially;
spectral analysis was performed on the combined system. Analysis of the
individual spectra was limited to regions where the contributions of A and B
can be disentangled. This analysis results in two lines of evidence indicating
that the X-ray emission from Hen 3-600 is derived from accretion processes:
line ratios of O VII indicate that the characteristic density of its
X-ray-emitting plasma is large; a significant component of low-temperature
plasma is present and is stronger in component A. These results are consistent
with results obtained from X-ray gratings spectroscopy of more rapidly
accreting systems. All of the signatures of Hen 3-600 that are potential
diagnostics of accretion activity -- X-ray emission, UV excess, H-alpha
emission, and weak infrared excess -- suggest that its components represent a
transition phase between rapidly accreting, classical T Tauri stars and
non-accreting, weak-lined T Tauri stars.Comment: latex, 27 pages, 12 figures, 6 tables; accepted by Ap
Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star
Astronomers have established that for a few million years newborn stars
possess disks of orbiting gas and dust. Such disks, which are likely sites of
planet formation, appear to disappear once these stars reach ages of 5-10 times
10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If
disks dissipate in less time than is needed for giant planet formation, such
planets may be rare and those known around nearby stars would be anomalies.
Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star
heretofore presumed nearly devoid of circumstellar material. We estimate that a
significant amount of H_2 persists in the gas phase, but only a tiny fraction
of this mass emits in the near-infrared. We propose that this star possesses an
evolved disk that has escaped detection thus far because much of the dust has
coagulated into planetesimals. This discovery suggests that the theory that
disks are largely absent around such stars should be reconsidered. The
widespread presence of such disks would indicate that planetesimals can form
quickly and giant planet formation can proceed to completion before the gas in
circumstellar disks disperses.Comment: latex 12 pages, including 1 figur
The GALEX nearby young-star survey
We describe a method that exploits data from the Galaxy Evolution Explorer (GALEX) ultraviolet and Wide-field Infrared Survey Explorer and Two Micron All Sky Survey infrared source catalogs, combined with proper motions and empirical pre-main sequence is
A New Method to Identify Nearby, Young, Low-mass Stars
We describe a new method to identify young, late-type stars within ~150 pc of
the Earth that employs visual or near-infrared data and the GALEX GR4/5
database. For spectral types later than K5, we demonstrate that the ratio of
GALEX near-ultraviolet (NUV) to visual and near-IR emission is larger for stars
with ages between 10 and 100 Myr than for older, main sequence stars. A search
in regions of the sky encompassing the TW Hya and Scorpius-Centaurus
Associations has returned 54 high-quality candidates for followup.
Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 angstrom
absorption in at least 17 systems. Because GALEX surveys have covered a
significant fraction of the sky, this methodology should prove valuable for
future young star studies.Comment: 27 pages, 8 figures, accepted for publication in ApJ. Some
significant changes were made in proof, we recommend readers use the ApJ
versio
Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogen gas
located in the circumstellar disks of six pre-main sequence stars, including
two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon
I star forming region. For two of these stars, we also place upper limits on
the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence
sources now known to be sources of quiescent near-infrared hydrogen emission,
four possess transitional disks, which suggests that detectable levels of H
emission and the presence of inner disk holes are correlated. These H
detections demonstrate that these inner holes are not completely devoid of gas,
in agreement with the presence of observable accretion signatures for all four
of these stars and the recent detections of [Ne II] emission from three of
them. The overlap in [Ne II] and H detections hints at a possible
correlation between these two features and suggests a shared excitation
mechanism of high energy photons. Our models, combined with the kinematic
information from the H lines, locate the bulk of the emitting gas at a few
tens of AU from the stars. We also find a correlation between H detections
and those targets which possess the largest H equivalent widths,
suggesting a link between accretion activity and quiescent H emission. We
conclude that quiescent H emission from relatively hot gas within the disks
of TTS is most likely related to on-going accretion activity, the production of
UV photons and/or X-rays, and the evolutionary status of the dust grain
populations in the inner disks.Comment: 12 pages, emulateapj, Accepted by Ap
Synaptic consolidation: from synapses to behavioral modeling
Synaptic plasticity, a key process for memory formation, manifests itself across different time scales ranging from a few seconds for plasticity induction up to hours or even years for consolidation and memory retention. We developed a three-layered model of synaptic consolidation that accounts for data across a large range of experimental conditions. Consolidation occurs in the model through the interaction of the synaptic efficacy with a scaffolding variable by a read-write process mediated by a tagging-related variable. Plasticity-inducing stimuli modify the efficacy, but the state of tag and scaffold can only change if a write protection mechanism is overcome. Our model makes a link from depotentiation protocols in vitro to behavioral results regarding the influence of novelty on inhibitory avoidance memory in rats
Investigating Star-disk Interactions During Late-stage Circusmtellar Disk Evolution in the Nearby Pre-MS Stars T Cha and TWA 30
We investigate, via contemporaneous X-ray and optical/IR observations, the nearby, pre-main sequence star/disk systems T Chamaeleontis (T Cha; D ~ 110 pc, age 3-5 Myr) and TWA 30A and 30B (D ~ 40 pc; age ~ 8 Myr). All three of these systems present opportunities to probe pre-main sequence (pre-MS) star-disk interactions during late-stage circumstellar disk evolution. The classical T Tauri star T Cha is the closest known example of a highly inclined, actively accreting, solar-mass star/disk system; furthermore, T Cha may be orbited by a low-mass companion or massive planet that has cleared an inner hole in its disk. We analyze near-simultaneous Chandra high-resolution X-ray and optical H-alpha spectroscopy observations of T Cha and find a correlation between X-ray and optical extinction resulting from variable photospheric obscuration from a disk warp/clump. We search for signatures of accretion and infer the X-ray absorbing properties of the T Cha circumstellar disk.We also present contemporaneous XMM-Newton X-ray and optical/IR spectroscopic observations of the nearby, actively accreting, very low-mass (mid-M) pre-MS star/disk/jet systems TWA 30A and 30B. Like T Cha, each component of this wide binary is viewed through a nearly edge-on circumstellar disk. We investigate potential X-ray accretion signatures, and compare the levels of magnetic activity in TWA 30A and 30B to those of other nearby, low-mass pre-MS stars near the H-burning limit. Both TWA 30A and 30B display large near-IR variability, suggestive of (respectively) variable obscuration of the stellar photosphere and a possible disk-rim warp. We detect only TWA 30A in X-rays and, similar to the case of T Cha, find a correlation between optical/IR and X-ray extinction associated with variable photospheric obscuration. The proximity and highly-inclined viewing geometries of the TWA 30 pair and T Cha, combined with contemporaneous optical/IR and X-ray observations, afford a unique opportunity to investigate the composition of late-stage circumstellar disks orbiting pre-MS stars
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