176 research outputs found
Oph 1622-2405: Not a Planetary-Mass Binary
We present an analysis of the mass and age of the young low-mass binary Oph
1622-2405. Using resolved optical spectroscopy of the binary, we measure
spectral types of M7.25+/-0.25 and M8.75+/-0.25 for the A and B components,
respectively. We show that our spectra are inconsistent with the spectral types
of M9 and M9.5-L0 from Jayawardhana & Ivanov and M9+/-0.5 and M9.5+/-0.5 from
Close and coworkers. Based on our spectral types and the theoretical
evolutionary models of Chabrier and Baraffe, we estimate masses of 0.055 and
0.019 Msun for Oph 1622-2405A and B, which are significantly higher than the
values of 0.013 and 0.007 Msun derived by Jayawardhana & Ivanov and above the
range of masses observed for extrasolar planets (M<=0.015 Msun). Planet-like
mass estimates are further contradicted by our demonstration that Oph
1622-2405A is only slightly later (by 0.5 subclass) than the composite of the
young eclipsing binary brown dwarf 2M 0535-0546, whose components have
dynamical masses of 0.034 and 0.054 Msun. To constrain the age of Oph
1622-2405, we compare the strengths of gravity-sensitive absorption lines in
optical and near-infrared spectra of the primary to lines in field dwarfs (>1
Gyr) and members of Taurus (~1 Myr) and Upper Scorpius (~5 Myr). The line
strengths for Oph 1622-2405A are inconsistent with membership in Ophiuchus (<1
Myr) and instead indicate an age similar to that of Upper Sco, which is
agreement with a similar analysis performed by Close and coworkers. We conclude
that Oph 1622-2405 is part of an older population in Sco-Cen, perhaps Upper Sco
itself.Comment: The Astrophysical Journal, in pres
The Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population
The census of Taurus–Auriga has been assembled over seven decades and inherited the biases and incompleteness of the input studies. The unusual shape of its inferred initial mass function (IMF) and the existence of isolated disk-bearing stars suggest that additional (likely disk-free) members remain to be discovered. We therefore have begun a global reassessment of the census of Taurus–Auriga that exploits new data and better definitions of youth and kinematic membership. As a first step, we reconsider the membership of all disk-free candidate members from the literature with spectral type ≥F0, 3^h50^m < α < 5^h40^m, and 14° < δ < 34°. We combine data from the literature with Keck/HIRES and UH88/SNIFS spectra to test the membership of these candidates using the positions in the Hertzsprung-Russel diagram, proper motions, radial velocities, Hα, lithium, and surface gravity. We find 218 confirmed or likely Taurus members, 160 confirmed or likely interlopers, and only 18 that lack sufficient evidence to draw firm conclusions. A significant fraction of these stars (81/218 = 37%) are not included in the most recent canonical member lists. There are few additional members to the immediate vicinity of the molecular clouds, preserving the IMFs that have been deemed anomalous in past work. Many of the likely Taurus members are instead distributed broadly across the search area. When combined with the known disk hosts, our updated census reveals two regimes: a high-density population with a high disk fraction (indicative of youth) that broadly traces the molecular clouds, and a low-density population with low disk fraction (hence likely older) that most likely represents previous generations of star formation
The Mass Function of Newly Formed Stars (Review)
The topic of the stellar "original mass function" has a nearly 50 year
history,dating to the publication in 1955 of Salpeter's seminal paper. In this
review I discuss the many more recent results that have emerged on the initial
mass function (IMF), as it is now called, from studies over the last decade of
resolved populations in star forming regions and young open clusters.Comment: 9 pages, 1 figure; to appear in "The Dense Instellar Medium in
Galaxies -- 4'th Cologne-Bonn-Zermatt-Symposium" editted by S. Pfalzner, C.
Kramer, C. Straubmeier and A. Heithausen, Springer-Verlag (2004
Exploring Halo Substructure with Giant Stars III: First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo
We describe first results of a spectroscopic probe of selected fields from
the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in
a strip of eleven fields along delta approximately -17^{circ}, in the range 12
<~ alpha <~ 17 hours, reveals a group of 8 giants that have kinematical
characteristics differing from the main field population, but that as a group
maintain coherent, smoothly varying distances and radial velocities with
position across the fields. Moreover, these stars have roughly the same
abundance, according to their MgH+Mgb absorption line strengths. Photometric
parallaxes place these stars in a semi-loop structure, arcing in a contiguous
distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial,
kinematical, and abundance coherence of these stars suggests that they are part
of a diffuse stream of tidal debris, and one roughly consistent with a wrapped,
leading tidal arm of the Sagittarius dwarf spheroidal galaxy.Comment: 8 pages including 4 figures. Accepted for publication in ApJ
A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391
Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and
the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed
an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc,
age~50Myr, solar metallicity). Here we present the results from our photometric
survey and from a spectroscopic follow-up of the central part of the survey.Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15
conferenc
Searching for Young M Dwarfs with GALEX
The census of young moving groups in the solar neighborhood is significantly
incomplete in the low-mass regime. We have developed a new selection process to
find these missing members based on the GALEX All-Sky Imaging Survey. For stars
with spectral types >K5 and younger than 300~Myr, we show that near-UV and
far-UV emission is greatly enhanced above the quiescent photosphere, analogous
to the enhanced X-ray emission of young low-mass stars seen by ROSAT but
detectable to much larger distances with GALEX. By combining GALEX data with
optical (HST Guide Star Catalog) and near-IR (2MASS) photometry, we identified
an initial sample of 34 young M dwarf candidates in a 1000 sq.~deg.~region
around the 10-Myr TW Hydra Association (TWA). Low-resolution spectroscopy of 30
of these found 16 which had H_alpha in emission, which were then followed-up at
high resolution to search for spectroscopic evidence of youth and to measure
radial velocities. Four objects have low surface gravities, photometric
distances and space motions consistent with TWA, but the non-detection of Li
indicates they may be too old to belong to this moving group. One object (M3.5,
93 pc) appears to be the first known accreting low-mass member of the 15~Myr
Lower Centaurus Crux OB association. Two objects exhibit all the
characteristics of the known TWA members, and thus we designate them as TWA 31
(M4.2, 110 pc) and TWA 32 (M6.3, 53 pc). TWA 31 shows extremely broad (447
km/s) H_alpha emission, making it the sixth member of TWA found to have ongoing
accretion. TWA 32 is resolved into a 0.6" binary in Keck laser guide star
adaptive optics imaging. Our search should be sensitive down to spectral types
of at least M4-M5 in TWA and thus the small numbers of new member is puzzling.
This may indicate TWA has an atypical mass function or that the presence of Li
may be too restrictive a criteria for selecting young low-mass stars.Comment: Accepted to Ap
Are There Age Spreads in Star Forming Regions?
A luminosity spread at a given effective temperature is ubiquitously seen in
the Hertzsprung-Russell (HR) diagrams of young star forming regions and often
interpreted in terms of a prolonged period (>=10 Myr) of star formation. I
review the evidence that the observed luminosity spreads are genuine and not
caused by astrophysical sources of scatter. I then address whether the
luminosity spreads necessarily imply large age spreads, by comparing HR diagram
ages with ages from independent clocks such as stellar rotation rate, the
presence of circumstellar material and lithium depletion. I argue that whilst
there probably is a true luminosity dispersion, there is little evidence to
support age spreads larger than a few Myr. This paradox could be resolved by
brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of
Large Surveys, 8 page
Thirty New Low-Mass Spectroscopic Binaries
As part of our search for young M dwarfs within 25 pc, we acquired
high-resolution spectra of 185 low-mass stars compiled by the NStars project
that have strong X-ray emission. By cross-correlating these spectra with radial
velocity standard stars, we are sensitive to finding multi-lined spectroscopic
binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of
27 SB2s, 2 SB3s and 1 SB4, increasing the number of known low-mass SBs by 50%
and proving that strong X-ray emission is an extremely efficient way to find
M-dwarf SBs. WASP photometry of 23 of these systems revealed two low-mass EBs,
bringing the count of known M dwarf EBs to 15. BD -22 5866, the SB4, is fully
described in Shkolnik et al. 2008 and CCDM J04404+3127 B consists of a two
mid-M stars orbiting each other every 2.048 days. WASP also provided rotation
periods for 12 systems, and in the cases where the synchronization time scales
are short, we used P_rot to determine the true orbital parameters. For those
with no P_rot, we use differential radial velocities to set upper limits on
orbital periods and semi-major axes. More than half of our sample has
near-equal-mass components (q > 0.8). This is expected since our sample is
biased towards tight orbits where saturated X-ray emission is due to tidal
spin-up rather than stellar youth. Increasing the samples of M dwarf SBs and
EBs is extremely valuable in setting constraints on current theories of stellar
multiplicity and evolution scenarios for low-mass multiple systems.Comment: Accepted to Ap
17 new very low-mass members in Taurus. The brown dwarf deficit revisited
Recent studies of the substellar population in the Taurus cloud have revealed
a deficit of brown dwarfs (BD) compared to the Trapezium cluster population
(Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However,
these works have concentrated on the highest stellar density regions of the
Taurus cloud. We have performed a large scale optical survey of this region,
covering a total area of 30 deg^2, and encompassing the densest part of the
cloud as well as their surroundings, down to a mass detection limits of 15
Jupiter Masses (MJ). In this paper, we present the optical spectroscopic
follow-up observations of 97 photometrically selected potential new low-mass
Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These
observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs.
Combining our observations with previously published results, we derive an
updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio
now appears consistent with the value previously derived in the Trapezium
cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication
that the relative numbers of BDs with respect to stars is decreased by a factor
2 in the central regions of the aggregates with respect to the more distributed
population. Our findings are best explained in the context of the
embryo-ejection model where brown dwarfs originate from dynamical interactions
in small N unstable multiple systems.Comment: 20 pages, 15 figure
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