172 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
A Large-Area Search for Low Mass Objects in Upper Scorpius I: The Photometric Campaign and New Brown Dwarfs
We present a wide-field photometric survey covering ~200 deg^2 toward the
Upper Scorpius OB association. Data taken in the R and I bands with the Quest-2
camera on the Palomar 48-inch telescope were combined with the 2MASS JHK survey
and used to select candidate pre-main sequence stars. Follow-up spectroscopy
with the Palomar 200-inch telescope of 62 candidate late-type members
identified 43 stars that have surface gravity signatures consistent with
association membership. From the optical/near-infrared photometry and derived
spectral types we construct an HR diagram for the new members and find 30
likely new brown dwarfs, nearly doubling the known substellar population of the
Upper Scorpius OB association. Continuation of our spectroscopic campaign
should reveal hundreds on new stellar and substellar members.Comment: 36 pages including 14 figures and 2 tables. Accepted for publication
in A
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
Astrometry with The \u3cem\u3eHubble Space Telescope\u3c/em\u3e: A Parallax of the Central Star of the Planetary Nebula NGC 6853
We present an absolute parallax and relative proper motion for the central star of the planetary nebula NGC 6853 (the Dumbbell). We obtain these with astrometric data from the Fine Guidance Sensor 3, a white-light interferometer on the Hubble Space Telescope. Spectral classifications and VRIJHKT2M and DDO51 photometry of the stars making up the astrometric reference frame provide spectrophotometric estimates of their absolute parallaxes. Introducing these into our model as observations with error, we find πabs = 2.10 ± 0.48 mas for the DAO central star of NGC 6853. A weighted average with a previous ground-based USNO determination yields πabs = 2.40 ± 0.32. We assume that the extinction suffered by the reference stars nearest (in angular separation and distance) to the central star is the same as for the central star. Correcting for color differences, we find AV = 0.30 ± 0.06 for the central star, hence, an absolute magnitude MV = 5.48. A recent determination of the central star effective temperature aided in estimating the central star radius, R* = 0.055 ± 0.02 R⊙, a star that may be descending to the white dwarf cooling track
Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members
We make use of new near and mid-IR photometry of the Pleiades cluster in
order to help identify proposed cluster members. We also use the new photometry
with previously published photometry to define the single-star main sequence
locus at the age of the Pleiades in a variety of color-magnitude planes.
The new near and mid-IR photometry extend effectively two magnitudes deeper
than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new
set of candidate very low mass and sub-stellar mass members of the Pleiades in
the central square degree of the cluster. We identify 42 new candidate members
fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should
eventually allow a better estimate of the cluster mass function to be made down
to of order 0.04 solar masses.
We also use new IRAC data, in particular the images obtained at 8 um, in
order to comment briefly on interstellar dust in and near the Pleiades. We
confirm, as expected, that -- with one exception -- a sample of low mass stars
recently identified as having 24 um excesses due to debris disks do not have
significant excesses at IRAC wavelengths. However, evidence is also presented
that several of the Pleiades high mass stars are found to be impacting with
local condensations of the molecular cloud that is passing through the Pleiades
at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available
at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07
A wide deep infrared look at the Pleiades with UKIDSS: new constraints on the substellar binary fraction and the low mass IMF
We present the results of a deep wide-field near-infrared survey of 12 square
degrees of the Pleiades conducted as part of the UKIDSS Deep Infrared Sky
Survey (UKIDSS) Galactic Cluster Survey (GCS). We have extracted over 340 high
probability proper motion members down to 0.03 solar masses using a combination
of UKIDSS photometry and proper motion measurements obtained by
cross-correlating the GCS with data from the Two Micron All Sky Survey (2MASS),
the Isaac Newton (INT) and the Canada-France-Hawai'i (CFHT) telescopes.
Additionally, we have unearthed 73 new candidate brown dwarf members on the
basis of five band UKIDSS photometry alone. We have identified 23 substellar
multiple system candidates out of 63 candidate brown dwarfs from the (Y-K,Y)
and (J-K,J) colour-magnitude diagrams, yielding a binary frequency of 28-44% in
the 0.075-0.030 Msun mass range. Our estimate is three times larger than the
binary fractions reported from high-resolution imaging surveys of field
ultracool dwarfs and Pleiades brown dwarfs. However, it is marginally
consistent with our earlier ``peculiar'' photometric binary fraction of
50+/-10% presented in Pinfield et al. (2003), in good agreement with the 32-45%
binary fraction derived from the recent Monte-Carlo simulations of Maxted &
Jeffries (2005) and compatible with the 26+/-10% frequency recently estimated
by Basri & Reiners (2006). A tentative estimate of the mass ratios from
photometry alone seems to support the hypothesis that binary brown dwarfs tend
to reside in near equal-mass ratio systems. (abridged)Comment: 21 pages, 8 figures, 6 tables, 1 electronic table, 6 appendices with
tables, accepted to MNRA
Predictors of residential stability among homeless young adults : a cohort study.
Abstract : BACKGROUND: Homelessness episodes have been shown to be associated with serious health outcomes among youth. This study was undertaken to estimate the probability of reaching residential stability over time and to identify predictors of residential stability among homeless young adults aged 18 to 25 years.
METHODS: A prospective cohort study was carried out in Montréal, Canada, between April 5(th) 2006 and January 21(th) 2009. Interviews conducted every three months included questions on life conditions and social and mental health factors that are known to influence residential trajectories. Residential status was determined, starting on the first day after recruitment; each follow-up day was classified as a homeless day or a housed day. A period of 90 days was used to define residential stability; therefore the main study outcome was the occurrence of the first consecutive 90 housed days during the follow-up period. Kaplan-Meier and Cox proportional-hazards regression analyses were conducted.
RESULTS: Of the 359 participants, 284 reached 90 days of residential stability over the study period, representing an annual probability of 80.5 %. In multivariate analysis, youth who had a high school degree, had a formal sector activity, and those who had sought psychological help were more likely to reach residential stability. Being a man, injecting substances, and having an informal sector activity were associated with a decreased probability to reach residential stability.
CONCLUSION: Exposure to factors related to opportunities that promote social integration increases the chance of reaching residential stability. On the other hand, factors related to high level of street entrenchment seem to interfere with stabilization. Maximum efforts should be made to prevent chronic homelessness among youth, targeting not only individual impairments but also hinging on services adapted to foster social connections among the youth
The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry
We report on the properties of pre-main-sequence objects in the Taurus
molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer
Space Telescope. There are 215 previously-identified members of the Taurus
star-forming region in our ~44 square degree map; these members exhibit a range
of Spitzer colors that we take to define young stars still surrounded by
circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no
detectable dust excesses). We looked for new objects in the survey field with
similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet
imaging, and found 148 candidate new members of Taurus. We have obtained
follow-up spectroscopy for about half the candidate sample, thus far confirming
34 new members, 3 probable new members, and 10 possible new members, an
increase of 15-20% in Taurus members. Of the objects for which we have
spectroscopy, 7 are now confirmed extragalactic objects, and one is a
background Be star. The remaining 93 candidate objects await additional
analysis and/or data to be confirmed or rejected as Taurus members. Most of the
new members are Class II M stars and are located along the same cloud filaments
as the previously-identified Taurus members. Among non-members with Spitzer
colors similar to young, dusty stars are evolved Be stars, planetary nebulae,
carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the
preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm
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