128 research outputs found
The very nearby M/T dwarf binary SCR 1845-6357
The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary
discovered. SCR 1845 is a particular object due to its tight orbit (currently
around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially
resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and
K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to
the primary SCR 1845A, orbital motion is evident even within a year. Following
the orbital motion, the binary's mass can be measured accurately within a
decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR
spectra allow for accurate determination of spectral type and also for rough
estimates of the object's physical parameters. The spectral type of SCR 1845B
is determined by direct comparison of the flux calibrated JHK spectra with T
dwarf standard template spectra and also by NIR spectral indices obtained from
synthetic photometry. Constrained values for surface gravity, effective
temperature and metallicity are derived by comparison with model spectra. Our
data prove that SCR 1845B is a brown dwarf of spectral type T6 that is
co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting
the NIR spectrum of SCR 1845B to model spectra yields an effective temperature
of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar
metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses
and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357
We investigate an XMM-Newton observation of SCR 1845-6357, a nearby,
ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM
detectors, however the X-ray emission is very likely from the M8.5 dwarf. We
compare its flaring emission to those of similar very low mass stars and
additionally present an XMM observation of the M8 dwarf VB 10. We detect
quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the
0.2-2.0 keV band, as well as a strong flare with a count rate increase of a
factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray
luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log
L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5
MK and frequent minor variability support this picture. During the flare, that
is accompanied by a significant brightening in the near-UV, plasma temperatures
of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is
reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at
detectable levels in quasi-quiescence, implying the existence of a corona. The
high activity level, coronal temperatures and the observed large flare point to
a rather active star, despite its estimated age of a few Gyr.Comment: Accepted by A&A, 6 pages, 5 figure
High-contrast spectroscopy of SCR J1845-6357 B
Spectral characterization of sub-stellar companions is essential to
understand their composition and formation processes. However, the large
contrast ratio of the brightness of each object to that of its parent star
limits our ability to extract a clean spectrum, free from any significant
contribution from the star. During the development of the long slit
spectroscopy (LSS) mode of IRDIS, the dual-band imager and spectrograph of
SPHERE, we proposed a data analysis method to estimate and remove the
contributions of the stellar spectrum. This method has never been tested on
real data because of the lack of instrumentation capable of combining adaptive
optics (AO), coronagraphy, and LSS. Nonetheless, a similar attenuation of the
star can be obtained using a particular observing configuration. Test data were
acquired using the AO-assisted spectrograph VLT/NACO. We obtained new J- and
H-band spectra of SCR J1845-6357 B, a T6 companion to a nearby (3.85\pm0.02 pc)
M8 star. This system is a well-suited benchmark as it is relatively wide
(~1.0") with a modest contrast ratio (~4 mag), and a previously published JHK
spectrum is available for reference. We demonstrate that (1) our method is
efficient at estimating and removing the stellar contribution, (2) it allows to
properly recover the spectral shape of the companion, and (3) it is essential
to obtain an unbiased estimation of physical parameters. We also show that the
slit configuration associated with this method allows us to use long exposure
times with high throughput producing high signal-to-noise ratio data. However,
the signal of the companion gets over-subtracted, particularly in our J-band
data, compelling us to use a fake companion spectrum to estimate and compensate
for the loss of flux. Finally, we report a new astrometric measurement of the
position of the companion (sep = 0.817", PA = 227.92 deg).Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in A&
The Solar Neighborhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star
We present the results of a multi-technique investigation of the M4.5Ve flare
star AP Col, which we discover to be the nearest pre-main-sequence star. These
include astrometric data from the CTIO 0.9m, from which we derive a proper
motion of 342.0+/-0.5 mas yr^-1, a trigonometric parallax of 119.21+/-0.98 mas
(8.39+/-0.07 pc), and photometry and photometric variability at optical
wavelengths. We also provide spectroscopic data, including radial velocity
(22.4+/-0.3 km s^-1), lithium Equivalent Width (EW) (0.28+/-0.02 A), H-alpha EW
(-6.0 to -35 A), {\it vsini} (11+/-1 km s^-1), and gravity indicators from the
Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle
spectrographs. The combined observations demonstrate that AP Col is the closer
of only two known systems within 10 pc of the Sun younger than 100 Myr. Given
its space motion and apparent age of 12-50 Myr, AP Col is likely a member of
the recently proposed ~40 Myr old Argus/IC 2391 association.Comment: 31 pages, 11 figure
Predictors of Long-term Exercise Maintenance among College Aged Adults: Role of Body Image Anxiety
Background and Purpose: Participation in regular exercise is low among young adults and is contributing to a rapid increase in obesity and chronic health conditions. Enhancing motivation is a key element in exercise initiation and maintenance. The current investigation considers factors relevant to the transtheoretical model (TTM), self-determination theory (SDT), self-efficacy (SE), and body image anxiety (BIA) in relation to college students’ motivation to exercise. Design and Main Outcome Measures: In this cross sectional study, lower division college students (N=614, 64% female, 36% male) completed an online survey of exercise behavior, motivation, SE and BIA. Results: BIA was related to both controlled extrinsic (external and introjected regulations) and autonomous extrinsic (integrated regulation) SDT motivational variables, as well as intrinsic motivation. Exercise maintenance was most strongly associated with integrated regulation, a “selfdetermined” motivational state, and SE. Conclusion: The current study provides support for the central tenet of SDT indicating that intrinsic and extrinsic motivation are not mutually exclusive constructs. Helping individuals with BIA develop a more intrinsic approach to exercise is integral for fostering long-term exercise maintenance. Thus, future research should focus on developing interventions that enhance integrated regulation and SE in order to promote exercise maintenance and reduce associated BIA
The Solar Neighborhood XXIII CCD Photometric Distance Estimates of SCR Targets -- 77 M Dwarf Systems within 25 Parsecs
We present CCD photometric distance estimates of 100 SCR (SuperCOSMOS RECONS)
systems with 0\farcs18/yr, 28 of which are new discoveries
previously unpublished in this series of papers. These distances are estimated
using a combination of new photometry acquired at CTIO and
magnitudes extracted from 2MASS. The estimates are improvements over those
determined using photographic plate magnitudes from SuperCOSMOS plus
, as presented in the original discovery papers. In total, 77 of the 100
systems investigated are predicted to be within 25 pc. If all 77 systems are
confirmed to have 40 milliarcseconds, this sample would
represent a 23% increase in M dwarf systems nearer than 25 pc in the southern
sky.Comment: 34 pages, 8 figure
Five new very low mass binaries
We report the discovery of companions to 5 nearby late M dwarfs (>M5),
LHS1901, LHS4009, LHS6167, LP869-26 and WT460, and we confirm that the recently
discovered mid-T brown dwarf companion to SCR1845-6357 is physically bound to
that star. These discoveries result from our adaptive optics survey of all M
dwarfs within 12 pc. The new companions have spectral types M5 to L1, and
orbital separations between 1 and 10 AU. They add significantly to the number
of late M dwarfs binaries in the immediate solar neighbourhood, and will
improve the multiplicity statistics of late M dwarfs. The expected periods
range from 3 to 130 years. Several pairs thus have good potential for accurate
mass determination in this poorly sampled mass range.Comment: 5 pages, 2 figures, submitted to Astronomy & Astrophysic
The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
We present the first results from our high-precision infrared (IR) astrometry
program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83
ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median
uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the
first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17
systems, we significantly improve upon published results, with a median (best)
improvement of 1.7x (5x). Three systems show astrometric perturbations
indicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and
2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). In
addition, we present here a large set of Keck adaptive optics imaging that more
than triples the number of binaries with L6--T5 components that have both
multi-band photometry and distances. Our data enable an unprecedented look at
the photometric properties of brown dwarfs as they cool through the L/T
transition. Going from \approxL8 to \approxT4.5, flux in the Y and J bands
increases by \approx0.7 mag and \approx0.5 mag, respectively (the Y- and J-band
"bumps"), while flux in the H, K, and L' bands declines monotonically. This
wavelength dependence is consistent with cloud clearing over a narrow range of
temperature, since condensate opacity is expected to dominate at 1.0--1.3
micron. Interestingly, despite more than doubling the near-IR census of L/T
transition objects, we find a conspicuous paucity of objects on the
color--magnitude diagram just blueward of the late-L/early-T sequence. This
"L/T gap" occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, and
implies that the last phases of cloud evolution occur rapidly. Finally, we
provide a comprehensive update to the absolute magnitudes of ultracool dwarfs
as a function of spectral type using a combined sample of 314 objects.Comment: Accepted to ApJ. New arXiv posting includes 4 new parallaxes and an
overall improvement in precision of 1.3x thanks to additional CFHT astrometry
for many targets. All data compiled in this paper (and more) are available
online: http://www.cfa.harvard.edu/~tdupuy/pl
The Brown Dwarf Kinematics Project (BDKP) I. Proper Motions and Tangential Velocities for a Large Sample of Late-type M, L and T Dwarfs
We report proper motion measurements for 427 late-type M, L and T dwarfs, 332
of which have been measured for the first time. Combining these new proper
motions with previously published measurements yields a sample of 841 M7-T8
dwarfs. We combined parallax measurements or calculated spectrophotometric
distances and computed tangential velocities for the entire sample. We find
that kinematics for the full and volume-limited 20 pc samples are consistent
with those expected for the Galactic thin disk, with no significant differences
between late-type M, L, and T dwarfs. Applying an age-velocity relation we
conclude that the average kinematic age of the 20 pc sample of ultracool dwarfs
is older than recent kinematic estimates and more consistent with age results
calculated with population synthesis models. There is a statistically distinct
population of high tangential velocity sources whose kinematics suggest an even
older population of ultracool dwarfs belonging to either the Galactic thick
disk or halo. We isolate subsets of the entire sample, including low
surface-gravity dwarfs, unusually blue L dwarfs, and photometric outliers in
J-Ks color and investigate their kinematics. We find that the spectroscopically
distinct class of unusually blue L dwarfs has kinematics clearly consistent
with old age, implying that high surface-gravity and/or low metallicity may be
relevant to their spectral properties. The low surface-gravity dwarfs are
kinematically younger than the overall population, and the kinematics of the
red and blue ultracool dwarfs suggest ages that are younger and older than the
full sample, respectively. We also present a reduced proper motion diagram at
2MASS Ks for the entire population and find that a limit of H_Ks > 18 excludes
M dwarfs from the L and T dwarf population regardless of near-infrared color.Comment: Accepted for publication in the Astronomical Journal, 21 pages text,
12 tables, 12 figure
Planets Around Low-Mass Stars (PALMS). I. A Substellar Companion to the Young M Dwarf 1RXS J235133.3+312720
We report the discovery of a brown dwarf companion to the young M dwarf 1RXS
J235133.3+312720 as part of a high contrast imaging search for planets around
nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.4"
(~120 AU) pair is confirmed to be comoving from two epochs of high resolution
imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of
1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of
L0. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV
activity levels comparable to young moving group members with ages of ~10-100
Myr. UVW kinematics based the measured radial velocity of the primary and the
system's photometric distance (50 +/- 10 pc) indicate it is likely a member of
the ~50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS
J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology
shows subtle hints of intermediate surface gravity. The spectrum is also an
excellent match to the ~200 Myr M9 brown dwarf LP 944-20. Assuming an age of
50-150 Myr, evolutionary models imply a mass of 32 +/- 6 Mjup for the
companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor
moving group after the L4 companion CD-35 2722 B and one of the few benchmark
brown dwarfs known at young ages.Comment: Accepted for publication in ApJ. 24 pages, 12 figures, 4 table
- …