1,216 research outputs found
High Resolution Near-Infrared Spectra of Protostars
We present new high resolution (R = 21,000) near-infrared (2 microns)
spectroscopic observations of a sample of Class I and flat-spectrum
protostellar objects in the rho Ophiuchi dark cloud. None of the five Class I
spectra show CO v = 0 -- 2 absorption features, consistent with high K-band
continuum veilings, 4 <= r_k <= 20 and fast stellar rotation, assuming that the
underlying protostellar photospheres are of late spectral type, as is suggested
by the low luminosities of most of these objects. Two of the flat-spectrum
protostellar objects also show no absorption features and are likely to be
highly veiled. The remaining two flat-spectrum sources show weak, broad
absorptions which are consistent with an origin in quickly rotating (v sin i ~
50 km / s) late-type stellar photospheres which are also strongly veiled, r_k =
3 - 4. These observations provide further evidence that: 1)-Class I sources are
highly veiled at near-infrared wavelengths, confirming previous findings of
lower resolution spectroscopic studies; and 2)- flat-spectrum protostars rotate
more rapidly than classical T Tauri stars (Class II sources), supporting
findings from a recent high resolution spectroscopic study of other
flat-spectrum sources in this cloud. In addition our observations are
consistent with the high rotation rates derived for two of the Class I
protostellar objects in our sample from observations of variable hard X-ray
emission obtained with the ASCA satellite. These observations suggest that
certain Class I sources can rotate even more rapidly than flat-spectrum
protostars, near breakup velocity.Comment: 16 pages including 2 tables and 2 figures (AASTeX 5.x) to be
published in The Astronomical Journal July 200
Using Chemistry to Unveil the Kinematics of Starless Cores: Complex Radial Motions in Barnard 68
We present observations of 13CO, C18O, HCO+, H13CO+, DCO+ and N2H+ line
emission towards the Barnard 68 starless core. The line profiles are
interpreted using a chemical network coupled with a radiative transfer code in
order to reconstruct the radial velocity profile of the core. Our observations
and modeling indicate the presence of complex radial motions, with the inward
motions in the outer layers of the core but outward motions in the inner part,
suggesting radial oscillations. The presence of such oscillation would imply
that B68 is relatively old, typically one order of magnitude older than the age
inferred from its chemical evolution and statistical core lifetimes. Our study
demonstrates that chemistry can be used as a tool to constrain the radial
velocity profiles of starless cores.Comment: 12 pages, 3 figures, to appear in the Astrophysical Journal Letter
A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution
We have carried out a large set of N-body simulations studying the effect of
residual-gas expulsion on the survival rate and final properties of star
clusters.
We have varied the star formation efficiency, gas expulsion timescale and
strength of the external tidal field, obtaining a three-dimensional grid of
models which can be used to predict the evolution of individual star clusters
or whole star cluster systems by interpolating between our runs. The complete
data of these simulations is made available on the Internet.
Our simulations show that cluster sizes, bound mass fraction and velocity
profile are strongly influenced by the details of the gas expulsion. Although
star clusters can survive star formation efficiencies as low as 10% if the
tidal field is weak and the gas is removed only slowly, our simulations
indicate that most star clusters are destroyed or suffer dramatic loss of stars
during the gas removal phase. Surviving clusters have typically expanded by a
factor 3 or 4 due to gas removal, implying that star clusters formed more
concentrated than as we see them today. Maximum expansion factors seen in our
runs are around 10. If gas is removed on timescales smaller than the initial
crossing time, star clusters acquire strongly radially anisotropic velocity
dispersions outside their half-mass radii. Observed velocity profiles of star
clusters can therefore be used as a constraint on the physics of cluster
formation.Comment: 12 pages, 9 figures, MNRAS accepte
Can Cascades be Predicted?
On many social networking web sites such as Facebook and Twitter, resharing
or reposting functionality allows users to share others' content with their own
friends or followers. As content is reshared from user to user, large cascades
of reshares can form. While a growing body of research has focused on analyzing
and characterizing such cascades, a recent, parallel line of work has argued
that the future trajectory of a cascade may be inherently unpredictable. In
this work, we develop a framework for addressing cascade prediction problems.
On a large sample of photo reshare cascades on Facebook, we find strong
performance in predicting whether a cascade will continue to grow in the
future. We find that the relative growth of a cascade becomes more predictable
as we observe more of its reshares, that temporal and structural features are
key predictors of cascade size, and that initially, breadth, rather than depth
in a cascade is a better indicator of larger cascades. This prediction
performance is robust in the sense that multiple distinct classes of features
all achieve similar performance. We also discover that temporal features are
predictive of a cascade's eventual shape. Observing independent cascades of the
same content, we find that while these cascades differ greatly in size, we are
still able to predict which ends up the largest
The impact of mass loss on star cluster formation. I. Analytic results
We study analytically the disruptive effect of instantaneous gas removal from
a cluster containing O stars.
We setup an iterative calculation based on the stellar velocity distribution
function to compute the fraction of stars that remain bound once the cluster
has ejected the gas and is out of equilibrium. We show that the stellar bound
fraction is a function of the initial cluster distribution function as well as
the star formation efficiency, , taken constant throughout the
cluster.
The case of the Plummer sphere is dealt with in greater details. We find for
this case that up to ~ 50% of the stars may remain bound when
assumes values < 1/2, contrary to expectations derived from the virial theorem.
The fraction of bound stars is expressed algebraically for polytropic
distribution functions.Comment: to appear in M
Spitzer observations of NGC2264: The nature of the disk population
NGC2264 is a young cluster with a rich circumstellar disk population which
makes it an ideal target for studying the evolution of stellar clusters. Our
goal is to study its star formation history and to analyse the primordial disk
evolution of its members. The study presented is based on data obtained with
Spitzer IRAC and MIPS, combined with deep NIR ground-based FLAMINGOS imaging
and previously published optical data. We build NIR dust extinction maps of the
molecular cloud associated with the cluster, and determine it to have a mass of
2.1x10^3Msun above an Av of 7mag. Using a differential K_s-band luminosity
function of the cluster, we estimate the size of its population to be
1436242 members. The star formation efficiency is ~25%. We identify the
disk population: (i) optically thick inner disks, (ii) anaemic inner disks, and
(iii) disks with inner holes, or transition disks. We analyse the spatial
distribution of these sources and find that sources with thick disks segregate
into sub-clusterings, whereas sources with anaemic disks do not. Furthermore,
sources with anaemic disks are found to be unembedded (Av<3mag), whereas the
clustered sources with thick disks are still embedded within the parental
cloud. NGC2264 has undergone more than one star-forming event, where the
anaemic and extincted thick disk population appear to have formed in separate
episodes. We also find tentative evidence of triggered star-formation in the
Fox Fur Nebula. In terms of disk evolution, our findings support the emerging
disk evolution paradigm of two distinct evolutionary paths for primordial
optically thick disks: a homologous one where the disk emission decreases
uniformly at NIR and MIR wavelengths, and a radially differential one where the
emission from the inner region of the disk decreases more rapidly than from the
outer region (forming transition disks).Comment: accepted for publishing in A&
Pholidosis Abnormalities and Injuries in the European Pond Turtle (Emys orbicularis) in the Conditions of the Khopersky Nature Reserve
Pholidosis abnormalities and injuries were studied through 194 specimens of European pond turtle (Emys orbicularis) in the Khopersky nature reserve (Voronezh Province, Russia) in 2008, 2009 and 2011. Six types of abnormalities (on the carapace only) were detected. The occurrence of individuals with all types of abnormalities, the partial occurrence of various abnormalities and the average number of abnormalities per individual were analyzed. Most individuals showed signs of predator attack, mostly on the carapace and tail. During winter hibernation, animals with limb injuries were noted
The Luminosity & Mass Function of the Trapezium Cluster: From B stars to the Deuterium Burning Limit
We use the results of a new, multi-epoch, multi-wavelength, near-infrared
census of the Trapezium Cluster in Orion to construct and to analyze the
structure of its infrared (K band) luminosity function. Specifically, we employ
an improved set of model luminosity functions to derive this cluster's
underlying Initial Mass Function (IMF) across the entire range of mass from OB
stars to sub-stellar objects down to near the deuterium burning limit. We
derive an IMF for the Trapezium Cluster that rises with decreasing mass, having
a Salpeter-like IMF slope until near ~0.6 M_sun where the IMF flattens and
forms a broad peak extending to the hydrogen burning limit, below which the IMF
declines into the sub-stellar regime. Independent of the details, we find that
sub-stellar objects account for no more than ~22% of the total number of likely
cluster members. Further, the sub-stellar Trapezium IMF breaks from a steady
power-law decline and forms a significant secondary peak at the lowest masses
(10-20 times the mass of Jupiter). This secondary peak may contain as many as
\~30% of the sub-stellar objects in the cluster. Below this sub-stellar IMF
peak, our KLF modeling requires a subsequent sharp decline toward the planetary
mass regime. Lastly, we investigate the robustness of pre-main sequence
luminosity evolution as predicted by current evolutionary models, and we
discuss possible origins for the IMF of brown dwarfs.Comment: 74 pages, 30 figures, AASTeX5.0. To be published in the 01 July 2002
ApJ. For color version of figure 1 and online data table see
http://www.astro.ufl.edu/~muench/PUB/publications.htm
Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar
We present the first spectrum of a highly veiled, strongly accreting
protostar which shows photospheric absorption features and demonstrates the
stellar nature of its central core. We find the spectrum of the luminous (L_bol
= 10 L_sun) protostellar source, YLW 15, to be stellar-like with numerous
atomic and molecular absorption features, indicative of a K5 IV/V spectral type
and a continuum veiling r_k = 3.0. Its derived stellar luminosity (3 L_sun) and
stellar radius (3.1 R_sun) are consistent with those of a 0.5 M_sun
pre-main-sequence star. However, 70% of its bolometric luminosity is due to
mass accretion, whose rate we estimate to be 1.6 E-6 M_sun / yr onto the
protostellar core. We determine that excess infrared emission produced by the
circumstellar accretion disk, the inner infalling envelope, and accretion
shocks at the surface of the stellar core of YLW 15 all contribute signifi-
cantly to its near-IR continuum veiling. Its projected rotation velocity v sin
i = 50 km / s is comparable to those of flat-spectrum protostars but
considerably higher than those of classical T Tauri stars in the rho Oph cloud.
The protostar may be magnetically coupled to its circumstellar disk at a radius
of 2 R_*. It is also plausible that this protostar can shed over half its
angular momentum and evolve into a more slowly rotating classical T Tauri star
by remaining coupled to its circumstellar disk (at increasing radius) as its
accretion rate drops by an order of magnitude during the rapid transition
between the Class I and Class II phases of evolution. The spectrum of WL 6 does
not show any photospheric absorption features, and we estimate that its
continuum veiling is r_k >= 4.6. Together with its low bolometric luminosity (2
L_sun), this dictates that its central core is very low mass, ~0.1 M_sun.Comment: 14 pages including 9 figures (3 figures of 3 panels each, all as
separate files). AASTeX LaTex macros version 5.0. To be published in The
Astronomical Journal (tentatively Oct 2002
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