We present an analysis of spectroscopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures toward the Orion Complex. By applying a hierarchical clustering algorithm to the ...six-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star-forming history within the Orion Complex and identify peculiar subclusters. With this method we reconstruct the older populations in the regions that are currently largely devoid of molecular gas, such as Orion C (which includes the Ori cluster) and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D group is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In λ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were halfway from the cluster center to their current positions. We also present a comparison between the parallax and proper-motion solutions obtained by Gaia DR2 and those obtained toward star-forming regions by the Very Long Baseline Array.
Close Companions around Young Stars Kounkel, Marina; Covey, Kevin; Moe, Maxwell ...
The Astronomical journal,
05/2019, Letnik:
157, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Multiplicity is a fundamental property that is set early during stellar lifetimes, and it is a stringent probe of the physics of star formation. The distribution of close companions around young ...stars is still poorly constrained by observations. We present an analysis of stellar multiplicity derived from Apache Point Observatory Galactic Evolution Experiment-2 spectra obtained in targeted observations of nearby star-forming regions. This is the largest homogeneously observed sample of high-resolution spectra of young stars. We developed an autonomous method to identify double-lined spectroscopic binaries (SB2s). Out of 5007 sources spanning the mass range of ∼0.05-1.5 M , we find 399 binaries, including both radial velocity (RV) variables and SB2s. The mass ratio distribution of SB2s is consistent with being uniform for q < 0.95 with an excess of twins for q > 0.95. The period distribution is consistent with what has been observed in close binaries (<10 au) in the evolved populations. Three systems are found to have q ∼ 0.1, with a companion located within the brown dwarf desert. There are no strong trends in the multiplicity fraction as a function of cluster age from 1 to 100 Myr. There is a weak dependence on stellar density, with companions being most numerous at * ∼ 30 stars/pc−2 and decreasing in more diffuse regions. Finally, disk-bearing sources are deficient in SB2s (but not RV variables) by a factor of ∼2; this deficit is recovered by the systems without disks. This may indicate a quick dispersal of disk material in short-period equal-mass systems that is less effective in binaries with lower q.
ABSTRACT
We present a uniform analysis of all mid-infrared R ≈ 90 spectra of field M5–T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The sample contains 113 spectra out of which 12 ...belong to late-M dwarfs, 69 to L dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the first time. We measure strengths of the main absorption bands in the IRS spectra, namely H2O at 6.25 $\rm{\mu m}$, CH4 at 7.65 $\rm{\mu m}$, NH3 at 10.5 $\rm{\mu m}$, and silicates over 8–11 $\rm{\mu m}$. Water absorption is present in all spectra and strengthens with spectral type. The onset of methane and ammonia occurs at the L8 and T2.5 types, respectively, although ammonia can be detectable as early as T1.5. Silicate absorption sets in at spectral type L2, is on average the strongest in L4–L6 dwarfs, and disappears past L8. However, silicate absorption can also be absent from the spectra at any L subtype. We find a positive correlation between the silicate absorption strength and the excess (deviation from median) near-infrared colour at a given L subtype, which supports the idea that variations of silicate cloud thickness produce the observed colour scatter in L dwarfs. We also find that variable L3–L7 dwarfs are twice more likely to have above-average silicate absorption than non-variables. The ensemble of results solidifies the evidence for silicate condensate clouds in the atmospheres of L dwarfs, and for the first time observationally establishes their emergence and sedimentation between effective temperatures of ≈2000 and ≈1300 K, respectively.
Abstract
Very young (
t
≲ 10 Myr) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar disks to the surface of the star. Upon impacting the stellar ...surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11–20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main-sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of
n
e
= 10
11
–10
12
cm
−3
, and excitation temperatures that are inversely correlated with electron density (from
T
∼ 5000 K for
n
e
∼ 10
12
cm
−3
to
T
∼ 12,500 K at
n
e
∼ 10
11
cm
−3
). These physical parameters are in good agreement with predictions from modeling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission-line sources in the APOGEE Data Release 17 data set.
Abstract
The Sloan Digital Sky Survey IV APOGEE-2 primary science goal was to observe red giant stars throughout the Galaxy to study its dynamics, morphology, and chemical evolution. The APOGEE ...instrument, a high-resolution 300-fiber
H
-band (1.55–1.71
μ
m) spectrograph, is also ideal to study other stellar populations in the Galaxy, among which are a number of star-forming regions and young open clusters. We present the results of the determination of six stellar properties (
T
eff
,
log
g
, Fe/H,
L
/
L
⊙
,
M
/
M
⊙
, and age) for a sample that is composed of 3360 young stars, of subsolar to supersolar types, in 16 Galactic star formation and young open cluster regions. Those sources were selected by using a clustering method that removes most of the field contamination. Samples were also refined by removing targets affected by various systematic effects of the parameter determination. The final samples are presented in a comprehensive catalog that includes all six estimated parameters. This overview study also includes parameter spatial distribution maps for all regions and Hertzsprung–Russell (
log
L
/
L
⊙
vs.
T
eff
) diagrams. This study serves as a guide for detailed studies on individual regions and paves the way for the future studies on the global properties of stars in the pre-main-sequence phase of stellar evolution using more robust samples.
Abstract
We present the discovery of CWISE J050626.96+073842.4 (CWISE J0506+0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 ...citizen science project. Photometry from UKIRT and CatWISE give a (
J
−
K
)
MKO
color of 2.97 ± 0.03 mag and a
J
MKO
− W2 color of 4.93 ± 0.02 mag, making CWISE J0506+0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506+0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity, late-type L/T transition dwarf. The spectrum of CWISE J0506+0738 shows possible signatures of CH
4
absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8
γ
–T0
γ
. We tentatively find that CWISE J0506+0738 is variable at 3–5
μ
m based on multiepoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the
β
Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.
Abstract
We present a large-scale study of stellar rotation for T Tauri stars in the Orion star-forming complex. We use the projected rotational velocity (
v
sin
(
i
)
) estimations reported by the ...APOGEE-2 collaboration as well as individual masses and ages derived from the position of the stars in the HR diagram, considering Gaia-EDR3 parallaxes and photometry plus diverse evolutionary models. We find an empirical trend for
v
sin
(
i
)
decreasing with age for low-mass stars (0.4
M
⊙
<
M
*
< 1.2
M
⊙
). Our results support the existence of a mechanism linking
v
sin
(
i
)
to the presence of accreting protoplanetary disks, responsible for regulating stellar rotation on timescales of about 6 Myr, which is the timescale in which most of the T Tauri stars lose their inner disk. Our results provide important constraints to models of rotation in the early phases of evolution of young stars and their disks.
ABSTRACT
Analysis of all archival 5–14 micron spectra of field ultracool dwarfs from the Infrared Spectrograph on the Spitzer Space Telescope has shown that absorption by silicates in the 8–11 micron ...region is seen in most L-type (1300 to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the atmospheres of L dwarfs and is strongest at L4–L6 spectral types. Herein we compare averages of the mid-infrared silicate absorption signatures of L3–L7 dwarfs that have low (≲104.5 cm s−2) versus high (≳105 cm s−2) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity, with young atmospheres having a broader, redder, and more asymmetric absorption profile. This indicates a difference in grain size and composition between dust condensates in young and old mid-L dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs matches expectations for ∼1 micron-sized amorphous iron- and magnesium-bearing pyroxene (MgxFe1 − xSiO3) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller (≲0.1 μm) and more volatile amorphous enstatite (MgSiO3) or SiO grains. This is the first direct spectroscopic evidence for gravity-dependent sedimentation of dust condensates in ultracool atmospheres. It confirms theoretical expectations for lower sedimentation efficiencies in low-gravity atmospheres and independently confirms their increased dustiness.
The Orion Star-forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. Existing membership catalogs span limited portions of the OSFC, reflecting the difficulty of selecting ...targets homogeneously across this extended, highly structured region. We have used data from wide-field photometric surveys to produce a less biased parent sample of young stellar objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar material or photometric variability at optical wavelengths across the full 420 square degree extent of the OSFC. When restricted to YSO candidates with H < 12.4, to ensure S/N ∼ 100 for a six-visit source, this uniformly selected sample includes 1307 IR excess sources selected using criteria vetted by Koenig & Liesawitz (2014) and 990 optical variables identified in the Pan-STARRS1 3π survey: 319 sources exhibit both optical variability and evidence of circumstellar disks through IR excess. Objects from this uniformly selected sample received the highest priority for targeting, but required fewer than half of the fibers on each APOGEE-2 plate. We filled the remaining fibers with previously confirmed and new color-magnitude selected candidate OSFC members. Radial velocity measurements from APOGEE-1 and new APOGEE-2 observations taken in the survey's first year indicate that ∼90% of the uniformly selected targets have radial velocities consistent with Orion membership. The APOGEE-2 Orion survey will include >1100 bona fide YSOs whose uniform selection function will provide a robust sample for comparative analyses of the stellar populations and properties across all sub-regions of Orion.
In this work, we have applied a semi-empirical spectral classification method for OB-stars using the APOGEE spectrograph to a sample of candidates in the W3-W4-W5 (W345) complexes. These massive ...star-forming regions span over 200 pc across the Perseus arm and have a notorious population of massive stars, from which a large fraction are members of various embedded and young open clusters. From 288 APOGEE spectra showing H-band spectral features typical of O- and B-type sources, 46 probably correspond to previously unknown O-type stars. Therefore, we confirm that Br11-Br13 together with He ii λ16923 (7-12) and He ii λ15723 (7-13) lines contained in the APOGEE spectral bands are useful in providing spectral classification down to one spectral sub-class for massive stars in regions as distant as d 2 kpc. The large number of newly found O-type stars as well as the numerous intermediate-mass population confirm that W345 is a very efficient massive star factory, with an integral stellar population probably amounting several thousand solar masses.
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