677 research outputs found
The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer
Aims. The brown dwarf (BD) formation process has not yet been completely
understood. To shed more light on the differences and similarities between star
and BD formation processes, we study and compare the disk fraction among both
kinds of objects over a large angular region in the Taurus cloud. In addition,
we examine the spatial distribution of stars and BD relative to the underlying
molecular gas Methods. In this paper, we present new and updated photometry
data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope
on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this
region. We also useed recently available CO mm data to study the spatial
distribution of stars and BDs relative to the cloud's molecular gas. Results.
We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a
value statistically consistent with the one among TTS (58 \pm 9%). We find that
BDs in transition from a state where they have a disk to a diskless state are
rare, and we study one isolated example of a transitional disk with an inner
radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR
excess compared to most members of Taurus that have disks. We find that BDs are
statistically found in regions of similar molecular gas surface density to
those associated with stars. Furthermore, we find that the gas column density
distribution is almost identical for stellar and substellar objects with and
without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics
The Effect of Controlled Photoperiod, Ergocryptine, and/or Melatonin on Reproduction in the Anestrous Ewe
The effect of decreased light and injections of ergocryptine (trial 1) or feeding of melatonin (trial 2) during the normal summer anestrous season of the ewe was studied. Decreased light (8 hr light:16 hr dark) and feeding of melatonin was effective in increasing the number of ewes cycling and conceiving. Decreased lighting also increased the total weight of all lambs born per ewe lambing compared to ewes in natural daylight during summer months. Ergocryptine appeared to have no effect on the ewe other than depressed prolactin levels
The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass
Upon its discovery in 2006, the young L7.5 companion to the solar analog HD
203030 was found to be unusual in being 200 K cooler than older late-L
dwarfs. HD 203030B offered the first clear indication that the effective
temperature at the L-to-T spectral type transition depends on surface gravity:
now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial
age analysis of the G8V primary star indicated that the system was 130--400 Myr
old, and so the companion between 12--31 . Using moderate
resolution near-infrared spectra of HD 203030B, we now find features of very
low gravity comparable to those of 10--150 Myr-old L7--L8 dwarfs. We also
obtained more accurate near infrared and {\sl Spitzer}/IRAC photometry, and
find a color of mag---comparable to those
observed in other young planetary-mass objects---and a luminosity of
log() dex. We further reassess the
evidence for the young age of the host star, HD 203030, with a more
comprehensive analysis of the photometry and updated stellar activity
measurements and age calibrations. Summarizing the age diagnostics for both
components of the binary, we adopt an age of 100 Myr for HD 203030B and an age
range of 30--150 Myr. Using cloudy evolutionary models, the new companion age
range and luminosity result in a mass of 11 with a range of 8--15
, and an effective temperature of K.Comment: 12 pages, 7 figures, accepted for publication in A
New Young Stars and Brown Dwarfs in the Upper Scorpius Association
To improve the census of the Upper Sco association (~11 Myr, ~145 pc), we
have identified candidate members using parallaxes, proper motions, and
color-magnitude diagrams from several wide-field imaging surveys and have
obtained optical and infrared spectra of several hundred candidates to measure
their spectral types and assess their membership. We also have performed
spectroscopy on a smaller sample of previously known or suspected members to
refine their spectral types and evidence of membership. We have classified 530
targets as members of Upper Sco, 377 of which lack previous spectroscopy. Our
new compilation of all known members of the association contains 1631 objects.
Although the census of Upper Sco has expanded significantly over the last
decade, there remain hundreds of candidates that lack spectroscopy. The precise
parallaxes and proper motions from the second data release of Gaia should
extend down to substellar masses in Upper Sco, which will greatly facilitate
the identification of the undiscovered members.Comment: Astronomical Journal, in press; machine readable tables and fits
spectra available at http://personal.psu.edu/kll207/usco.ta
Near-infrared Detection of WD 0806-661 B with the Hubble Space Telescope
WD 0806-661 B is one of the coldest known brown dwarfs (T=300-345 K) based on
previous mid-infrared photometry from the Spitzer Space Telescope. In addition,
it is a benchmark for testing theoretical models of brown dwarfs because its
age and distance are well-constrained via its primary star (2+/-0.5 Gyr,
19.2+/-0.6 pc). We present the first near-infrared detection of this object,
which has been achieved through F110W imaging (~Y+J) with the Wide Field Camera
3 on board the Hubble Space Telescope. We measure a Vega magnitude of
m110=25.70+/-0.08, which implies J~25.0. When combined with the Spitzer
photometry, our estimate of J helps to better define the empirical sequence of
the coldest brown dwarfs in M4.5 versus J-[4.5]. The positions of WD 0806-661 B
and other Y dwarfs in that diagram are best matched by the cloudy models of
Burrows et al. and the cloudless models of Saumon et al., both of which employ
chemical equilibrium. The calculations by Morley et al. for 50% cloud coverage
differ only modestly from the data. Spectroscopy would enable a more stringent
test of the models, but based on our F110W measurement, such observations are
currently possible only with Hubble, and would require at least ~10 orbits to
reach a signal-to-noise ratio of ~5
Testing Theoretical Evolutionary Models with AB Dor C and the Initial Mass Function
We assess the constraints on the evolutionary models of young low-mass
objects that are provided by the measurements of the companion AB Dor C by
Close and coworkers and by a new comparison of model-derived IMFs of
star-forming regions to the well-calibrated IMF of the solar neighborhood.
After performing an independent analysis of Close's imaging and spectroscopic
data for AB Dor C, we find that AB Dor C is not detected at a significant level
(SN 1.2) in the SDI images when one narrow-band image is subtracted from
another, but that it does appear in the individual SDI frames as well as the
images at JHK. Using the age of 75-150 Myr for AB Dor from Luhman, Stauffer, &
Mamajek, the luminosity predicted by the models of Chabrier & Baraffe is
consistent with the value that we estimate. We measure a spectral type of
M6+/-1 from the K-band spectrum of AB Dor C, which is earlier than the value of
M8+/-1 from Close and is consistent with the model predictions when a dwarf
temperature scale is adopted. In a test of these models at much younger ages,
we show that the low-mass IMFs that they produce for star-forming regions are
similar to the IMF of the solar neighborhood. If the masses of the low-mass
stars and brown dwarfs in these IMFs of star-forming regions were
underestimated by a factor of two as suggested by Close, then the IMF
characterizing the current generation of Galactic star formation would have to
be radically different from the IMF of the solar neighborhood.Comment: 15 pages, accepted to the Astrophysical Journa
Unusually Wide Binaries: Are They Wide or Unusual?
We describe an astrometric and spectroscopic campaign to confirm the youth
and association of a complete sample of candidate wide companions in Taurus and
Upper Sco. Our survey found fifteen new binary systems (3 in Taurus and 12 in
Upper Sco) with separations of 3-30" (500-5000 AU) among all of the known
members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in
these two regions conforms to only some expectations from field multiplicity
surveys. Higher-mass stars have a higher frequency of wide binary companions,
and there is a marked paucity of wide binary systems near the substellar
regime. However, the separation distribution appears to be log-flat, rather
than declining as in the field, and the mass ratio distribution is more biased
toward similar-mass companions than the IMF or the field G dwarf distribution.
The maximum separation also shows no evidence of a limit at <5000 AU until the
abrupt cessation of any wide binary formation at system masses of ~0.3 Msun. We
attribute this result to the post-natal dynamical sculpting that occurs for
most field systems; our binary systems will escape to the field intact, but
most field stars are formed in denser clusters and do not. In summary, only
wide binary systems with total masses <0.3 Msun appear to be "unusually wide".Comment: Accepted to ApJ; 23 pages, 9 figures, 9 tables in emulateapj. Table 6
is online-only and is attached as tab6.te
Optical spectroscopy of X-ray sources in the Taurus molecular cloud: discovery of ten new pre-main sequence stars
We have analyzed optical spectra of 25 X-ray sources identified as potential
new members of the Taurus molecular cloud (TMC), in order to confirm their
membership in this SFR. Fifty-seven candidates were previously selected among
the X-ray sources in the XEST survey, having a 2MASS counterpart compatible
with a PMS star based on color-magnitude and color-color diagrams. We obtained
high-resolution optical spectra for 7 of these candidates with the SARG
spectrograph at the TNG telescope, which were used to search for Li absorption
and to measure the Ha line and the radial and rotational velocities; 18
low-resolution optical spectra obtained with DOLORES for other candidate
members were used for spectral classification, for Ha measurements, and to
assess membership together with IR color-color and color-magnitude diagrams and
additional information from the X-ray data. We found that 3 sources show Li
absorption, with equivalent widths of ~500 mA, broad spectral line profiles,
indicating v sin i ~20-40 km/s, radial velocities consistent with those for
known members, and Ha emission. Two of them are classified as new WTTSs, while
the EW (~ -9 Ang) of the Ha line and its broad asymmetric profile clearly
indicate that the third star (XEST-26-062) is a CTTS. Fourteen sources observed
with DOLORES are M-type stars. Fifteen sources show Ha emission; 6 of them have
spectra that indicate surface gravity lower than in MS stars, and their
de-reddened positions in IR color-magnitude diagrams are consistent with their
derived spectral type and with PMS models at the distance of the TMC. The
K-type star XEST-11-078 is confirmed as a new member from the strength of its
Ha emission line. Overall, we confirm membership to the TMC for 10 out of 25
X-ray sources observed in the optical. Three sources remain uncertain.Comment: 15 pages, 7 figures, accepted by Astronomy & Astrophysic
Comparing Three Bee Species for Controlled Pollination of Selected Brassicaceae
Pollination of species of Brassicaceae for seed increase at the USDA-ARS North Central Regional Plant Introduction Station (NCRPIS) has been accomplished for several years by using nucleus hives of honey bees, Apis mellifera L., in field cages. Brassicaceae are cool season crops that need pollination from late April to early June in the north temperate zone. Overwintered hives of honey bees need time to strengthen (i.e., increase their numbers) and thus it is difficult to make enough nuclei to meet our early season pollinations needs. Purchasmg package bees from suppliers in the southern U.S. is an expensive alternative. In this study, three bee species; a solitary bee, Osmza cornifrons (Radoszkowski) (Hymenoptera: Megachilidae); honey bees; and alfalfa leafcutting bees, Megachile rotundata (Fab) (Hymenoptera: Megachilidae), were compared for their utility as early season pollinators of Brassicaceae species in field cages. Osmza cornifrons proved to be equal to or better than leafcutters and honey bees for pollination of Brassicaceae in field cages as measured by seed produced/plant. Future seed increases of Brassicaceae at the NCRPIS will utilize Osmia cornifrons in field cages because they are effective pollinators and they are easily managed
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