4,776 research outputs found
Rotation in the Orion Nebula Cluster
Eighteen fields in the Orion Nebula Cluster (ONC) have been monitored for one
or more observing seasons from 1990-99 with a 0.6-m telescope at Wesleyan
University. Photometric data were obtained in Cousins I on 25-40 nights per
season. Results from the first 3 years of monitoring were analyzed by Choi &
Herbst (1996; CH). Here we provide an update based on 6 more years of
observation and the extensive optical and IR study of the ONC by Hillenbrand
(1997) and Hillenbrand et al. (1998). Rotation periods are now available for
134 ONC members. Of these, 67 were detected at multiple epochs with identical
periods by us and 15 more were confirmed by Stassun et al. (1999) in their
study of Ori OBIc/d. The bimodal period distribution for the ONC is confirmed,
but we also find a clear dependence of rotation period on mass. This can be
understood as an effect of deuterium burning, which temporarily slows the
contraction and thus spin-up of stars with M <0.25 solar masses and ages of ~1
My. Stars with M <0.25 solar masses have not had time to bridge the gap in the
period distribution at ~4 days. Excess H-K and I-K emission, as well as CaII
infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but
significant correlations with rotation period among stars with M >0.25 solar
masses. Our results provide new observational support for the importance of
disks in the early rotational evolution of low mass stars. [abridged]Comment: 18 pages of text, 17 figures, and 4 tables; accepted for publication
in The Astronomical Journa
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
Rotation of Late-Type Stars in Praesepe with K2
We have Fourier analyzed 941 K2 light curves of likely members of Praesepe,
measuring periods for 86% and increasing the number of rotation periods (P) by
nearly a factor of four. The distribution of P vs. (V-K), a mass proxy, has
three different regimes: (V-K)<1.3, where the rotation rate rapidly slows as
mass decreases; 1.3<(V-K)<4.5, where the rotation rate slows more gradually as
mass decreases; and (V-K)>4.5, where the rotation rate rapidly increases as
mass decreases. In this last regime, there is a bimodal distribution of
periods, with few between 2 and 10 days. We interpret this to mean
that once M stars start to slow down, they do so rapidly. The K2 period-color
distribution in Praesepe (790 Myr) is much different than in the Pleiades
(125 Myr) for late F, G, K, and early-M stars; the overall distribution
moves to longer periods, and is better described by 2 line segments. For mid-M
stars, the relationship has similarly broad scatter, and is steeper in
Praesepe. The diversity of lightcurves and of periodogram types is similar in
the two clusters; about a quarter of the periodic stars in both clusters have
multiple significant periods. Multi-periodic stars dominate among the higher
masses, starting at a bluer color in Praesepe ((V-K)1.5) than in the
Pleiades ((V-K)2.6). In Praesepe, there are relatively more light curves
that have two widely separated periods, 6 days. Some of these could
be examples of M star binaries where one star has spun down but the other has
not.Comment: Accepted by Ap
Rotation of Low-mass Stars in Taurus with K2
We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young (~3 Myr) Taurus association, in addition to an older (~30 Myr) population of stars that is largely in the foreground of the Taurus molecular clouds. Out of 156 of the highest-confidence Taurus members, we find that 81% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all stars (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation toward both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than ~2 days. The overall amplitude of the LCs decreases with age, and higher-mass stars have generally lower amplitudes than lower-mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting LC morphologies are also different between these two classes
Study on the neuronal circuits implicated in postural tremor and hypokinesia
The effect of various tegmentary lesions at the level of the pontomesenchphalon in monkeys on motor function was observed. The importance of the monoaminergic mechanisms of the brainstem is discussed. The results also show the importance of the descending tegmentary rubral system and the rubroolivocerebellar circuit in controlling peripheral motor activity. The destruction of the sensory motor cortex proves to be a more effective way of eliminating spontaneous or harmaline induced tremor than the complete interruption of the pyramidal system on the level of the cerebral peduncle
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
A Census of the Young Cluster IC 348
We present a new census of the stellar and substellar members of the young
cluster IC 348. We have obtained images at I and Z for a 42'x28' field
encompassing the cluster and have combined these measurements with previous
optical and near-infrared photometry. From spectroscopy of candidate cluster
members appearing in these data, we have identified 122 new members, 15 of
which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015
M_sun by recent evolutionary models. The latest census for IC 348 now contains
a total of 288 members, 23 of which are later than M6 and thus are likely to be
brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a
16'x14' field centered on the cluster, we construct an IMF that is unbiased in
mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where
the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses
down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and
then declines into the substellar regime. In comparison, the similarly-derived
IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak
near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of
the IMFs in IC 348 and Taurus are reflected in the distributions of spectral
types, which peak at M5 and K7, respectively. These data provide compelling,
model-independent evidence for a significant variation of the IMF with
star-forming conditions.Comment: 47 pages, 14 figures, 3rd para of 4.5.3 has been added, this is final
version in press at ApJ, also found at
http://cfa-www.harvard.edu/sfgroup/preprints.htm
The Seven Sisters DANCe III: Projected spatial distribution
Methods. We compute Bayesian evidences and Bayes Factors for a set of
variations of the classical radial models by King (1962), Elson et al. (1987)
and Lauer et al. (1995). The variations incorporate different degrees of model
freedom and complexity, amongst which we include biaxial (elliptical) symmetry,
and luminosity segregation. As a by-product of the model comparison, we obtain
posterior distributions and maximum a posteriori estimates for each set of
model parameters. Results. We find that the model comparison results depend on
the spatial extent of the region used for the analysis. For a circle of 11.5
parsecs around the cluster centre (the most homogeneous and complete region),
we find no compelling reason to abandon Kings model, although the Generalised
King model, introduced in this work, has slightly better fitting properties.
Furthermore, we find strong evidence against radially symmetric models when
compared to the elliptic extensions. Finally, we find that including mass
segregation in the form of luminosity segregation in the J band, is strongly
supported in all our models. Conclusions. We have put the question of the
projected spatial distribution of the Pleiades cluster on a solid probabilistic
framework, and inferred its properties using the most exhaustive and least
contaminated list of Pleiades candidate members available to date. Our results
suggest however that this sample may still lack about 20% of the expected
number of cluster members. Therefore, this study should be revised when the
completeness and homogeneity of the data can be extended beyond the 11.5
parsecs limit. Such study will allow a more precise determination of the
Pleiades spatial distribution, its tidal radius, ellipticity, number of objects
and total mass.Comment: 39 pages, 31 figure
Cluster membership probabilities from proper motions and multiwavelength photometric catalogues: I. Method and application to the Pleiades cluster
We present a new technique designed to take full advantage of the high
dimensionality (photometric, astrometric, temporal) of the DANCe survey to
derive self-consistent and robust membership probabilities of the Pleiades
cluster. We aim at developing a methodology to infer membership probabilities
to the Pleiades cluster from the DANCe multidimensional astro-photometric data
set in a consistent way throughout the entire derivation. The determination of
the membership probabilities has to be applicable to censored data and must
incorporate the measurement uncertainties into the inference procedure.
We use Bayes' theorem and a curvilinear forward model for the likelihood of
the measurements of cluster members in the colour-magnitude space, to infer
posterior membership probabilities. The distribution of the cluster members
proper motions and the distribution of contaminants in the full
multidimensional astro-photometric space is modelled with a
mixture-of-Gaussians likelihood. We analyse several representation spaces
composed of the proper motions plus a subset of the available magnitudes and
colour indices. We select two prominent representation spaces composed of
variables selected using feature relevance determination techniques based in
Random Forests, and analyse the resulting samples of high probability
candidates. We consistently find lists of high probability (p > 0.9975)
candidates with 1000 sources, 4 to 5 times more than obtained in the
most recent astro-photometric studies of the cluster.
The methodology presented here is ready for application in data sets that
include more dimensions, such as radial and/or rotational velocities, spectral
indices and variability.Comment: 14 pages, 4 figures, accepted by A&
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