503 research outputs found
Epsilon Indi Ba/Bb: the nearest binary brown dwarf
We have carried out high angular resolution near-infrared imaging and
low-resolution (R~1000) spectroscopy of the nearest known brown dwarf, Eps Indi
B, using the ESO VLT NAOS/CONICA adaptive optics system. We find it to be a
close binary (as also noted by Volk et al. 2003) with an angular separation of
0.732 arcsec, corresponding to 2.65AU at the 3.626pc distance of the Eps Indi
system. In our discovery paper (Scholz et al. 2003), we concluded that Eps Indi
B was a ~50Mjup T2.5 dwarf: our revised finding is that the two system
components (Eps Indi Ba and Eps Indi Bb) have spectral types of T1 and T6,
respectively, and estimated masses of 47 and 28Mjup, respectively, assuming an
age of 1.3Gyr. Errors in the masses are +/-10 and +/-7Mjup, respectively,
dominated by the uncertainty in the age determination (0.8-2Gyr range). This
uniquely well-characterised T dwarf binary system should prove important in the
study of low-mass, cool brown dwarfs. The two components are bright and
relatively well-resolved: Eps Indi B is the only T dwarf binary in which
spectra have been obtained for both components. They have a well-established
distance and age. Finally, their orbital motion can be measured on a fairly
short timescale (nominal orbital period 15 yrs), permitting an accurate
determination of the true total system mass, helping to calibrate brown dwarf
evolutionary models.Comment: Accepted for publication by Astronomy & Astrophysics main journal.
This replacement version includes minor changes made following comments by
the referee, along with a reworking of the photometric data and derived
quantities using 2MASS catalogue photometry as the basis, with only a minor
impact on the final result
Behavioural variability, physical activity, rumination time, and milk characteristics of dairy cattle in response to regrouping
We gratefully thank the farm staff and the Lely Farm Management Support Ireland and UK manager, Mr Bas Van Santen, for technical assistance and data retrieval from the Lely T4C software database and Dr Sharon Mitchell for assistance with the laboratory analyses. This work has already been published as part of a PhD thesis (Marumo et al. (2021a)).Peer reviewe
The Distribution of Stellar Mass in the Pleiades
As part of an effort to understand the origin of open clusters, we present a
statistical analysis of the currently observed Pleiades. Starting with a
photometric catalog of the cluster, we employ a maximum likelihood technique to
determine the mass distribution of its members, including single stars and both
components of binary systems. We find that the overall binary fraction for
unresolved pairs is 68%. Extrapolating to include resolved systems, this
fraction climbs to about 76%, significantly higher than the accepted field-star
result. Both figures are sensitive to the cluster age, for which we have used
the currently favored value of 125 Myr. The primary and secondary masses within
binaries are correlated, in the sense that their ratios are closer to unity
than under the hypothesis of random pairing. We map out the spatial variation
of the cluster's projected and three-dimensional mass and number densities.
Finally, we revisit the issue of mass segregation in the Pleiades. We find
unambiguous evidence of segregation, and introduce a new method for quantifying
it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa
New Rotation Periods in the Open Cluster NGC 1039 (M 34), and a Derivation of its Gyrochronology Age
Employing photometric rotation periods for solar-type stars in NGC 1039 [M
34], a young, nearby open cluster, we use its mass-dependent rotation period
distribution to derive the cluster's age in a distance independent way, i.e.,
the so-called gyrochronology method. We present an analysis of 55 new rotation
periods,using light curves derived from differential photometry, for solar type
stars in M 34. We also exploit the results of a recently-completed,
standardized, homogeneous BVIc CCD survey of the cluster in order to establish
photometric cluster membership and assign B-V colours to each photometric
variable. We describe a methodology for establishing the gyrochronology age for
an ensemble of solar-type stars. Empirical relations between rotation period,
photometric colour and stellar age (gyrochronology) are used to determine the
age of M 34. Based on its position in a colour-period diagram, each M 34 member
is designated as being either a solid-body rotator (interface or I-star), a
differentially rotating star (convective or C-star) or an object which is in
some transitory state in between the two (gap or g-star). Fitting the period
and photometric colour of each I-sequence star in the cluster, we derive the
cluster's mean gyrochronology age.
47/55 of the photometric variables lie along the loci of the cluster main
sequence in V/B-V and V/V-I space. We are further able to confirm kinematic
membership of the cluster for half of the periodic variables [21/55], employing
results from an on-going radial velocity survey of the cluster. For each
cluster member identified as an I-sequence object in the colour-period diagram,
we derive its individual gyrochronology age, where the mean gyro age of M 34 is
found to be 193 +/- 9 Myr, formally consistent (within the errors) with that
derived using several distance-dependent, photometric isochrone methods (250
+/- 67 Myr).Comment: accepted for publication in Astronomy & Astrophysic
The Mass and Structure of the Pleiades Star Cluster from 2MASS
We present the results of a large scale search for new members of the
Pleiades star cluster using 2MASS near-infrared photometry and proper motions
derived from POSS plates digitized by the USNO PMM program. The search extends
to a 10 degree radius around the cluster, well beyond the presumed tidal
radius, to a limiting magnitude of R ~ 20, corresponding to ~ 0.07 M_sun at the
distance and age of the Pleiades. Multi-object spectroscopy for 528 candidates
verifies that the search was extremely effective at detecting cluster stars in
the 1 - 0.1 M_sun mass range using the distribution of H_alpha emission
strengths as an estimate of sample contamination by field stars.
When combined with previously identified, higher mass stars, this search
provides a sensitive measurement of the stellar mass function and dynamical
structure of the Pleiades. The degree of tidal elongation of the halo agrees
well with current N body simulation results. Tidal truncation affects masses
below ~ 1 M_sun. The cluster contains a total mass ~ 800 M_sun. Evidence for a
flatter mass function in the core than in the halo indicates the depletion of
stars in the core with mass less than ~ 0.5 M_sun, relative to stars with mass
\~1 - 0.5 M_sun, and implies a preference for very low mass objects to populate
the halo or escape. The overall mass function is best fitted with a lognormal
form that becomes flat at ~ 0.1 M_sun. Whether sufficient dynamical evaporation
has occurred to detectably flatten the initial mass function, via preferential
escape of very low mass stars and brown dwarfs, is undetermined, pending better
membership information for stars at large radial distances.Comment: 19 pages, 14 figures, 2 tables, accepted by AJ, to appear April 200
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
2MASS Galaxies in the Fornax Cluster Spectroscopic Survey
The Fornax Cluster Spectroscopic Survey (FCSS) is an all-object survey of a
region around the Fornax Cluster of galaxies undertaken using the 2dF
multi-object spectrograph on the Anglo-Australian Telescope. Its aim was to
obtain spectra for a complete sample of all objects with 16.5 < b_j < 19.7
irrespective of their morphology (i.e. including `stars', `galaxies' and
`merged' images). We explore the extent to which (nearby) cluster galaxies are
present in 2MASS. We consider the reasons for the omission of 2MASS galaxies
from the FCSS and vice versa. We consider the intersection (2.9 square degrees
on the sky) of our data set with the infra-red 2 Micron All-Sky Survey (2MASS),
using both the 2MASS Extended Source Catalogue (XSC) and the Point Source
Catalogue (PSC). We match all the XSC objects to FCSS counterparts by position
and also extract a sample of galaxies, selected by their FCSS redshifts, from
the PSC. We confirm that all 114 XSC objects in the overlap sample are
galaxies, on the basis of their FCSS velocities. A total of 23 Fornax Cluster
galaxies appear in the matched data, while, as expected, the remainder of the
sample lie at redshifts out to z = 0.2 (the spectra show that 61% are early
type galaxies, 18% are intermediate types and 21% are strongly star
forming).The PSC sample turns out to contain twice as many galaxies as does the
XSC. However, only one of these 225 galaxies is a (dwarf) cluster member. On
the other hand, galaxies which are unresolved in the 2MASS data (though almost
all are resolved in the optical) amount to 71% of the non-cluster galaxies with
2MASS detections and have redshifts out to z=0.32.Comment: 5 pages, accepted by A&A, resubmitted due to missing reference
Deep, Wide-field CCD Photometry for the Open Cluster NGC3532
We present the results of a deep, wide-field CCD survey for the open cluster
NGC~3532. Our new photometry effectively covers a one square
degree area and reaches an unprecedented depth of to reveal that
NGC~3532 is a rich open cluster that harbors a large number of faint, low-mass
stars. We employ a number of methods to reduce the impact of field star
contamination in the cluster color-magnitude diagrams, including supplementing
our photometry with data from the 2MASS catalog. These efforts allow
us to define a robust sample of candidate main sequence stars suitable for a
purely empirical determination of the cluster's parameters by comparing them to
the well-established Hyades main sequence. Our results confirm previous
findings that NGC~3532 lies fairly near to the Sun [;
~pc] and has an extremely low reddening for its location near
the Galactic plane []. Moreover, an age of \,Myr
has been derived for the cluster by fitting a set of overshooting isochrones to
the well-populated upper main-sequence. This new photometry also extends faint
enough to reach the cluster white dwarf sequence, as confirmed by our
photometric recovery of eight spectroscopically identified members of the
cluster. Using the location of these eight members, along with the latest
theoretical cooling tracks, we have identified additional white dwarf
stars in the color-magnitude diagram that have a high probability
of belonging to NGC~3532. The age we derive from fitting white dwarf isochrones
to the locus of these stars, \,Myr, is consistent with the age
derived from the turnoff. Our analysis of the photometry also includes an
estimation of the binary star fraction, as well as a determination of the
cluster's luminosity and mass functions.Comment: Accepted for publication in AJ. 55 pages, 21 figures. High-quality
version with complete data tables can be downloaded from
http://www.phys.lsu.edu/~jclem/NGC3532
Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster
We present near-infrared photometry and optical spectroscopy of very low-mass
stars and brown dwarf candidates in the Pleiades open cluster. The membership
status of these objects is assessed. Eight objects out of 45 appear to be
non-members. A search for companions among 34 very low-mass Pleiades members
(M0.09 M) in high-spatial resolution images obtained with the
Hubble Space Telescope and the adaptive optics system of the
Canada-France-Hawaii telescope produced no resolved binaries with separations
larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find
evidence for a binary sequence in the color-magnitude diagrams, in agreement
with the results of Steele & Jameson (1995) for higher mass stars. We compare
the multiplicity statistics of the Pleiades very low-mass stars and brown
dwarfs with that of G and K-type main sequence stars in the solar neighborhood
(Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency
of wide binary systems (separation >27 AU) among the Pleiades very low-mass
members. We briefly discuss how this result can fit with current scenarios of
brown dwarf formation. We correct the Pleiades substellar mass function for the
contamination of cluster non-members found in this work. We find a
contamination level of 33% among the brown dwarf candidates identified by
Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary,
we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass
bin is still rising but the contribution to the total mass of the cluster is
declining in the brown dwarf regime.Comment: to be published in The Astrophysical Journa
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