1,651 research outputs found
The Spectroscopically Determined Substellar Mass Function of the Orion Nebula Cluster
We present a spectroscopic study of candidate brown dwarf members of the
Orion Nebula Cluster (ONC). We obtained new J- and/or K-band spectra of ~100
objects within the ONC which are expected to be substellar based on their
K,(H-K) magnitudes and colors. Spectral classification in the near-infrared of
young low mass objects is described, including the effects of surface gravity,
veiling due to circumstellar material, and reddening. From our derived spectral
types and existing near-infrared photometry we construct an HR diagram for the
cluster. Masses are inferred for each object and used to derive the brown dwarf
fraction and assess the mass function for the inner 5.'1 x 5.'1 of the ONC,
down to ~0.02 solar masses. The derived logarithmic mass function rises to a
peak at ~0.2 solar masses, similar to previous IMF determinations derived from
purely photometric methods, but falls off more sharply at the hydrogen-burning
limit before leveling through the substellar regime. We compare the mass
function derived here for the inner ONC to those presented in recent literature
for the sparsely populated Taurus cloud members and the rich cluster IC 348. We
find good agreement between the shapes and peak values of the ONC and IC 348
mass distributions, but little similarity between the ONC and Taurus results.Comment: Accepted for Publication in Apj. Added Erratu
A multi-color optical survey of the orion nebula cluster. II. The H-R diagram
We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical
photometry and spectroscopy.We study the color–color diagrams in BVI, plus a narrowband filter centered at 6200 Å, finding evidence that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M
spectral-type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main-sequence isochrone.We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrowband photometry; in this way, we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BVI color–color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice of also the spectral-type–temperature transformation, we produce the new Hertzsprung–Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ~2 to ~3 Myr depending on the model. We study the distribution of the members in the mass–age plane and find that taking into account selection effects due to incompleteness,
removes an apparent correlation between mass and age.We derive the initial mass function for low- and intermediate mass members of the ONC, which turns out to be model dependent and shows a turnover at M ≲ 0.2 M_⊙
The photometric evolution of dissolving star clusters I: First predictions
We calculated the broad-band photometric evolution of unresolved star
clusters, including the preferential loss of low-mass stars due to mass
segregation. The stellar mass function of a cluster evolves due to three
effects: (a) the evolution of massive stars; (b) early tidal effects reduce the
mass function independently of the stellar mass; (c) after mass segregation has
completed, tidal effects preferentially remove the lowest-mass stars from the
cluster. Results: (1) During the first ~40% of the lifetime of a cluster the
cluster simply gets fainter due to the loss of stars by tidal effects. (2)
Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of
low-mass stars. This will result in an underestimate of the age of clusters if
standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of
the total lifetime of a cluster it will rapidly get redder. This is because
stars at the low-mass end of the main sequence, which are preferentially lost,
are bluer than the AGB stars that dominate the light at long wavelengths,
resulting in an age overestimate. (4) Clusters with mass segregation and the
preferential loss of low-mass stars evolve along almost the same tracks in
colour-colour diagrams as clusters without mass segregation. Therefore it will
be difficult to distinguish this effect from that due to the cluster age for
unresolved clusters, unless the total lifetime of the clusters can be
estimated. (5) The changes in the colour evolution of unresolved clusters due
to the preferential loss of low-mass stars will affect the determination of the
SFHs. (6) The preferential loss of low-mass stars might explain the presence of
old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as
intermediate-age clusters (2 - 5 Gyr). [Abridged]Comment: accepted for publication in A&
Persistency of Priors-Induced Bias in Decision Behavior and the fMRI Signal
It is well known that people take advantage of prior knowledge to bias decisions. To investigate this phenomenon behaviorally and in the brain, we acquired fMRI data while human subjects viewed ambiguous abstract shapes and decided whether a shape was of Category A (smoother) or B (bumpier). The decision was made in the context of one of two prior knowledge cues, 80/20 and 50/50. The 80/20 cue indicated that upcoming shapes had an 80% probability of being of one category, e.g., B, and a 20% probability of being of the other. The 50/50 cue indicated that upcoming shapes had an equal probability of being of either category. The ideal observer would bias decisions in favor of the indicated alternative at 80/20 and show zero bias at 50/50. We found that subjects did bias their decisions in the predicted direction at 80/20 but did not show zero bias at 50/50. Instead, at 50/50 the subjects retained biases of the same sign as their 80/20 biases, though of diminished magnitude. The signature of a persistent though diminished bias at 50/50 was also evident in fMRI data from frontal and parietal regions previously implicated in decision-making. As a control, we acquired fMRI data from naïve subjects who experienced only the 50/50 stimulus distributions during both the pre-scan training and the fMRI experiment. The behavioral and fMRI data from the naïve subjects reflected decision biases closer to those of the ideal observer than those of the prior knowledge subjects at 50/50. The results indicate that practice making decisions in the context of non-equal prior probabilities biases decisions made later when prior probabilities are equal. This finding may be related to the “anchoring and adjustment” strategy described in the psychology, economics, and marketing literatures, in which subjects adjust a first approximation response – the “anchor” – based on additional information, typically applying insufficient adjustment relative to the ideal observer
Binary Stars in the Orion Nebula Cluster
We report on a high-spatial-resolution survey for binary stars in the
periphery of the Orion Nebula Cluster, at 5 - 15 arcmin (0.65 - 2 pc) from the
cluster center. We observed 228 stars with adaptive optics systems, in order to
find companions at separations of 0.13" - 1.12" (60 - 500 AU), and detected 13
new binaries. Combined with the results of Petr (1998), we have a sample of 275
objects, about half of which have masses from the literature and high
probabilities to be cluster members. We used an improved method to derive the
completeness limits of the observations, which takes into account the elongated
point spread function of stars at relatively large distances from the adaptive
optics guide star. The multiplicity of stars with masses >2 M_sun is found to
be significantly larger than that of low-mass stars. The companion star
frequency of low-mass stars is comparable to that of main-sequence M-dwarfs,
less than half that of solar-type main-sequence stars, and 3.5 to 5 times lower
than in the Taurus-Auriga and Scorpius-Centaurus star-forming regions. We find
the binary frequency of low-mass stars in the periphery of the cluster to be
the same or only slightly higher than for stars in the cluster core (<3 arcmin
from theta1C Ori). This is in contrast to the prediction of the theory that the
low binary frequency in the cluster is caused by the disruption of binaries due
to dynamical interactions. There are two ways out of this dilemma: Either the
initial binary frequency in the Orion Nebula Cluster was lower than in
Taurus-Auriga, or the Orion Nebula Cluster was originally much denser and
dynamically more active.Comment: 20 page
The extinction map of the OMC-1 molecular cloud behind the Orion Nebula
Our main goal is to derive a new extinction map of the OMC-1, obtaining
information about the structure of the OMC-1 and the Orion Nebula Cluster. The
most recent near-infrared catalog of stars is used to study the distribution of
reddening across a ~0.3 deg^2 area covering the Orion Nebula Cluster. On the
basis of the observed (H,H-K_S) diagram, we establish a criterion for
disentangling contaminants from bona-fide cluster members. For contaminant
stars, interstellar reddenings are estimated by comparison with a synthetic
galactic model. A statistical analysis is then performed to consistently
account for local extinction, reddening and star-counts analysis. We derive the
extinction map of the OMC-1 with angular resolution <5'. We also assemble a
sample of candidate cluster members, for which we measure the extinction
provided by the nebular environment. These extinction measurements are analyzed
similarly to the contaminant sample, and an extinction map of the Orion Nebula
is derived. The extinction provided by the OMC-1 is variable on spatial scales
of a few arcminutes, while showing a general increase from the outskirts
(A_V~6) to the direction of the Trapezium asterism (A_V>30). The Orion Nebula
extinction map is more irregular and optically thinner, with A_V of the order
of a few magnitudes. Both maps are consistent with the optical morphology, in
particular the Dark Bay to the north-east of the Trapezium. Both maps also show
the presence of a north-south high-density ridge, which confirms the
filamentary structure of the Orion molecular complex inside which star
formation is still taking place.Comment: Published in the electronic edition of Astronomy & Astrophysics. It
is available on http://dx.doi.org/10.1051/0004-6361/20111655
A Young Globular Cluster in the Galaxy NGC 6946
A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11
pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by
clouds of dust and smaller young clusters inside a giant circular bubble 300 pc
in radius. At the edge of the bubble is an arc of regularly-spaced clusters
that could have been triggered during the bubble's formation. The region is at
the end of a spiral arm, suggesting an origin by the asymmetric collapse of
spiral arm gas. The globular is one of the nearest examples of a cluster that
is similar to the massive old globulars in the Milky Way. We consider the
energetics of the bubble and possible formation mechanisms for the globular
cluster, including the coalescence of smaller clusters.Comment: 20 pages, 7 figures, accepted for Astrophysical Journal Vol 535, June
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The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit
We present a new census of the Orion Nebula Cluster (ONC) over a large field
of view (>30'x30'), significantly increasing the known population of stellar
and substellar cluster members with precisely determined properties. We develop
and exploit a technique to determine stellar effective temperatures from
optical colors, nearly doubling the previously available number of objects with
effective temperature determinations in this benchmark cluster. Our technique
utilizes colors from deep photometry in the I-band and in two medium-band
filters at lambda~753 and 770nm, which accurately measure the depth of a
molecular feature present in the spectra of cool stars. From these colors we
can derive effective temperatures with a precision corresponding to better than
one-half spectral subtype, and importantly this precision is independent of the
extinction to the individual stars. Also, because this technique utilizes only
photometry redward of 750nm, the results are only mildly sensitive to optical
veiling produced by accretion. Completing our census with previously available
data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign
masses and ages down to 0.02 solar masses. At faint luminosities, we detect a
large population of background sources which is easily separated in our
photometry from the bona fide cluster members. The resulting initial mass
function of the cluster has good completeness well into the substellar mass
range, and we find that it declines steeply with decreasing mass. This suggests
a deficiency of newly formed brown dwarfs in the cluster compared to the
Galactic disk population.Comment: 16 pages, 18 figures. Accepted for publication in The Astrophysical
Journa
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