258 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
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 Distributed Population of Low Mass Pre-Main Sequence Stars near the Taurus Molecular Clouds
We present a drift scan survey covering a ~5 deg by 50 deg region toward the
southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I
filters with the Quest-2 camera on the Palomar 48-inch telescope were combined
with 2MASS near-infrared photometry to select candidate young stars. Follow-up
optical spectroscopy of 190 candidates led to identification of 42 new low mass
pre-main sequence stars with spectral types M4-M8, of which approximately half
exhibit surface gravity signatures similar to known Taurus stars while the
other half exhibit surface gravity signatures similar to members of the
somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main
sequence stars are spread over ~35 deg, and many are located well outside of
previously explored regions. From assessment of the spatial and proper motion
distributions, we argue that the new pre-main sequence stars identified far
from the clouds cannot have originated from the vicinity of the 1-2 Myr-old
subclusters which contain the bulk of the identified Taurus members, but
instead represent a newly-identified area of recent star-formation near the
clouds.Comment: Accepted for publication in AJ. 13 pages including 9 figures (2 in
color) and 1 table. A separate file tabA1.ps contains a hard copy of a second
table which will be published in electronic form onl
Population analysis of open clusters: radii and mass segregation
Aims: Based on our well-determined sample of open clusters in the all-sky
catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and
investigate the effect of mass segregation of stars in open clusters. Methods:
Using statistical methods, we study the distribution of linear sizes as a
function of spatial position and cluster age. We also examine statistically the
distribution of stars of different masses within clusters as a function of the
cluster age. Results: No significant dependence of the cluster size on location
in the Galaxy is detected for younger clusters (< 200 Myr), whereas older
clusters inside the solar orbit turned out to be, on average, smaller than
outside. Also, small old clusters are preferentially found close to the
Galactic plane, whereas larger ones more frequently live farther away from the
plane and at larger Galactocentric distances. For clusters with (V - M_V) <
10.5, a clear dependence of the apparent radius on age has been detected: the
cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age
of 1 Gyr. A detailed analysis shows that this observed effect can be explained
by mass segregation and does not necessarily reflect a real decrease of cluster
radii. We found evidence for the latter for the majority of clusters older than
30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some
clusters with a significant grade of mass segregation, whereas some others show
no segregation at all. At a cluster age between 50 and 100 Myrs, the
distribution of stars of different masses becomes more regular over cluster
area. In older clusters the evolution of the massive stars is the most
prominent effect we observe.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
The distance to the Orion Nebula Cluster
The distance to the Orion Nebula Cluster (ONC) is estimated using the
rotational properties of its low-mass pre main-sequence (PMS) stars. Rotation
periods, projected equatorial velocities and distance-dependent radius
estimates are used to form an observational sin i distribution (where i is the
axial inclination), which is modelled to obtain the distance estimate. A
distance of 440+/-34 pc is found from a sample of 74 PMS stars with spectral
types between G6 and M2, but this falls to 392+/-32 pc when PMS stars with
accretion discs are excluded on the basis of their near-infrared excess. Since
the radii of accreting stars are more uncertain and probably systematically
underestimated, then this closer distance is preferred. The quoted
uncertainties include statistical errors and uncertainties due to a number of
systematic effects including binarity and inclination bias. This method is
geometric and independent of stellar evolution models, though does rely on the
assumption of random axial orientations and the Cohen & Kuhi (1979) effective
temperature scale for PMS stars. The new distance is consistent with, although
lower and more precise, than most previous ONC distance estimates. A closer ONC
distance implies smaller luminosities and an increased age based on the
positions of PMS stars in the Hertzsprung-Russell diagram.Comment: Accepted for publication in MNRAS (12 pages) Table 1 available from
the autho
Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Sco
We have obtained multi-fibre intermediate-resolution optical spectroscopy of
94 photometric and proper motion selected low-mass star and brown dwarf
candidates in Upper Sco with AAT/AAOmega. We have estimated the spectral types
and measured the equivalent widths of youth and gravity diagnostic features to
confirm the spectroscopic membership of about 95% of the candidates extracted
from 6.5 square degrees in Upper Sco. We also detect lithium in the spectra
with the highest signal-to-noise, consolidating our conclusions about their
youth. Furthermore, we derive an estimate of our selections using spectroscopic
data obtained for a large number of stars falling into the instrument's
field-of-view. We have estimated the effective temperatures and masses for each
new spectroscopic member using the latest evolutionary models available for
low-mass stars and brown dwarfs. Combining the current optical spectroscopy
presented here with near-infrared spectroscopy obtained for the faintest
photometric candidates, we confirm the shape and slope of our earlier
photometric mass function. The luminosity function drawn from the spectroscopic
sample of 113 USco members peaks at around M6 and is flat at later spectral
type. We may detect the presence of the M7/M8 gap in the luminosity function as
a result of the dust properties in substellar atmospheres. The mass function
may peak at 0.2 Msun and is quite flat in the substellar regime. We observe a
possible excess of cool low-mass brown dwarfs compared to IC 348 and the
extrapolation of the field mass functions, supporting the original hypothesis
that Upper Sco may possess an excess of brown dwarfs. This result shows that
the selection of photometric candidates based on five band photometry available
from the UKIDSS GCS and complemented partially by proper motions can lead to a
good representation of the spectroscopic mass function (abridged).Comment: Accepted by A&A: 18 pages with 9 figures and 3 tables in main, 2
figures and 4 tables in Appendices. Some tables in electronic forma
Quantitative Evidence for an Intrinsic Age Spread in the Orion Nebula Cluster
Aims. We present a study of the distribution of stellar ages in the Orion
Nebula Cluster (ONC) based on accurate HST photometry taken from the HST
Treasury Program observations of the ONC utilizing the most recent estimate of
the cluster's distance (Menten et al. 2007). We investigate the presence of an
intrinsic age spread in the region and a possible trend of age with the spatial
distribution. Methods. We estimate the extinction and accretion luminosity
towards each source by performing synthetic photometry on an empirical
calibration of atmospheric models (Da Rio et al. 2010) using the package
Chorizos (Maiz-Apellaniz 2004). The position of the sources in the HR-diagram
is compared with different theoretical isochrones to estimate the mean cluster
age and age dispersion. Through Monte Carlo simulations we quantify the amount
of intrinsic age spread in the region, taking into account uncertainties on the
distance, spectral type, extinction, unresolved binaries, accretion and
photometric variability. Results. According to Siess et al. (2000) evolutionary
models the mean age of the Cluster is 2.2 Myr with a scatter of few Myrs. With
Monte Carlo simulations we find that the observed age spread is inconsistent
with a coeval stellar population, but is in agreement with a star formation
activity between 1.5 and 3.5 Myrs. We also observe light evidence for a trend
of ages with spatial distribution.Comment: 12 pages, 12 figures, Accepted for publication in Astronomy and
Astrophysic
No wide spread of stellar ages in the Orion Nebula Cluster
The wide luminosity dispersion seen for stars at a given effective
temperature in the H-R diagrams of young clusters and star forming regions is
often interpreted as due to significant (~10 Myr) spreads in stellar
contraction age. In the scenario where most stars are born with circumstellar
discs, and that disc signatures decay monotonically (on average) over
timescales of only a few Myr, then any such age spread should lead to clear
differences in the age distributions of stars with and without discs. We have
investigated large samples of stars in the Orion Nebula Cluster (ONC) using
three methods to diagnose disc presence from infrared measurements. We find no
significant difference in the mean ages or age distributions of stars with and
without discs, consistent with expectations for a coeval population. Using a
simple quantitative model we show that any real age spread must be smaller than
the median disc lifetime. For a log-normal age distribution, there is an upper
limit of <0.14 dex (at 99% confidence) to any real age dispersion, compared to
the ~=0.4 dex implied by the H-R diagram. If the mean age of the ONC is 2.5
Myr, this would mean at least 95% of its low-mass stellar population has ages
between 1.3--4.8 Myr. We suggest that the observed luminosity dispersion is
caused by a combination of observational uncertainties and physical mechanisms
that disorder the conventional relationship between luminosity and age for pre
main-sequence stars. This means that individual stellar ages from the H-R
diagram are unreliable and cannot be used to directly infer a star formation
history. Irrespective of what causes the wide luminosity dispersion, the
finding that any real age dispersion is less than the median disc lifetime
argues strongly against star formation scenarios for the ONC lasting longer
than a few Myr.Comment: To appear in MNRAS, 13 page
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