1,472 research outputs found
A dense micro-cluster of Class 0 protostars in NGC 2264 D-MM1
We present sensitive and high angular resolution (~1") 1.3 mm continuum
observations of the dusty core D-MM1 in the Spokes cluster in NGC 2264 using
the Submillimeter Array. A dense micro-cluster of seven Class 0 sources was
detected in a 20" x 20" region with masses between 0.4 to 1.2 solar masses and
deconvolved sizes of about 600 AU. We interpret the 1.3 mm emission as arising
from the envelopes of the Class 0 protostellar sources. The mean separation of
the 11 known sources (SMA Class 0 and previously known infrared sources) within
D-MM1 is considerably smaller than the characteristic spacing between sources
in the larger Spokes cluster and is consistent with hierarchical thermal
fragmentation of the dense molecular gas in this region.Comment: Accepted for publication in the Astrophysical Journal Letter
The [alpha/Fe] Ratios in Dwarf Galaxies: Evidence for a Non-universal Stellar Initial Mass Function?
It is well established that the [alpha/Fe] ratios in elliptical galaxies
increase with galaxy mass. This relation holds also for early-type dwarf
galaxies, although it seems to steepen at low masses. The [alpha/Fe] vs. mass
relation can be explained assuming that smaller galaxies form over longer
timescales (downsizing), allowing a larger amount of Fe (mostly produced by
long-living Type Ia Supernovae) to be released and incorporated into newly
forming stars. Another way to obtain the same result is by using a flatter
initial mass function (IMF) in large galaxies, increasing in this way the
number of Type II Supernovae and therefore the production rate of
alpha-elements. The integrated galactic initial mass function (IGIMF) theory
predicts that the higher the star formation rate, the flatter the IMF. We have
checked, by means of semi-analytical calculations, that the IGIMF theory,
combined with the downsizing effect (i.e. the shorter duration of the star
formation in larger galaxies), well reproduces the observed [alpha/Fe] vs. mass
relation. In particular, we show a steepening of this relation in dwarf
galaxies, in accordance with the available observations.Comment: 4 pages, 2 figures; to appear in the proceedings of the JENAM 2010
Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010
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&
Algebra Structures on Hom(C,L)
We consider the space of linear maps from a coassociative coalgebra C into a
Lie algebra L. Unless C has a cocommutative coproduct, the usual symmetry
properties of the induced bracket on Hom(C,L) fail to hold. We define the
concept of twisted domain (TD) algebras in order to recover the symmetries and
also construct a modified Chevalley-Eilenberg complex in order to define the
cohomology of such algebras
A 2MASS Analysis of the Stability of Southern Bok Globules
We used near-infrared 2MASS data to construct visual extinction maps of a
sample of Southern Bok globules utilizing the NICE method. We derived radial
extinction profiles of dense cores identified in the globules and analyzed
their stability against gravitational collapse with isothermal Bonnor-Ebert
spheres. The frequency distribution of the stability parameter xi_max of these
cores shows that a large number of them are located in stable states, followed
by an abrupt decrease of cores in unstable states. This decrease is steeper for
globules with associated IRAS point sources than for starless globules.
Moreover, globules in stable states have a Bonnor-Ebert temperature of T = 15
+- 6 K, while the group of critical plus unstable globules has a different
temperature of T = 10 +- 3 K. Distances were estimated to all the globules
studied in this work and the spectral class of the IRAS sources was calculated.
No variations were found in the stability parameters of the cores and the
spectral class of their associated IRAS sources. On the basis of 13CO J = 1-0
molecular line observations, we identified and modeled a blue-assymetric line
profile toward a globule of the sample, obtaining an upper limit infall speed
of 0.25 km/s.Comment: 53 pages, 15 figures, accepted for publication in Ap
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
A multi-wavelength census of stellar contents in the young cluster NGC 1624
We present a comprehensive multi-wavelength analysis of the young cluster NGC
1624 associated with the H II region Sh2-212 using optical UBVRI photometry,
optical spectroscopy and GMRT radio continuum mapping along with the
near-infrared (NIR) JHK archival data. Reddening E(B-V) and distance to the
cluster are estimated to be 0.76 - 1.00 mag and 6.0 +/- 0.8 kpc, respectively.
Present analysis yields a spectral class of O6.5V for the main ionizing source
of the region. The distribution of YSOs in (J-H)/ (H-K) NIR colour-colour
diagram shows that a majority of them have A_V 4 mag. Based on the NIR
excess characteristics, we identified 120 probable candidate YSOs in this
region which yield a disk frequency of ~ 20%. These YSOs are found to have an
age spread of ~ 5 Myr with a median age of ~ 2-3 Myr and a mass range of ~ 0.1
- 3.0 . A significant number of YSOs are located close to the cluster
centre and we detect an enhanced density of reddened YSOs located/projected
close to the molecular clumps at the periphery of NGC 1624. This indicates that
the YSOs located within the cluster core are relatively older in comparison to
those located/projected near the clumps. From the radio continuum flux,
spectral class of the ionizing source of the ultra-compact H II region at the
periphery of Sh2-212 is estimated to be ~ B0.5V. From optical data, slope of
the mass function (MF) , in the mass range can
be represented by a single power law with a slope -1.18 +/- 0.10, whereas the
NIR data in the mass range yields = -1.31
+/- 0.15. The slope of the K-band luminosity function (KLF) for the cluster is
found to be 0.30 +/- 0.06 which is in agreement with the values obtained for
other young clusters.Comment: Accepted for publication in MNRA
Near infrared imaging of NGC2316
In the present paper we present JHK photometric results of the young embedded
cluster NGC 2316. We construct the cluster radial profile from which we
determine a radius of 0.63 pc. We find 189 29 cluster members in an
extinction limited sub-sample of the survey, 22 19 of which are possibly
substellar. An average extinction of 4.5 visual magnitudes is derived using
(H-K) colours of control fields. This extinction is due to the presence of
residual parental molecular cloud. NGC 2316 presents 16% source fraction of
excess emission which is consistent with other results from clusters with an
age of 2-3 Myr. This age is consistent with the distribution of sources in the
colour-magnitude diagram when compared to theoretical isochrones, and the
overall shape of the cluster KLF. The substellar population of the cluster is
similar or smaller than that observed for other embedded clusters and the
stellar objects dominate the cluster membership.Comment: 4 pages, 5 figures, accepted for publication in Astronomy &
Astrophysics Letters. Full resolution images and paper available from
http://www-cfa.harvard.edu/~pteixeir/NGC2316
The CMF as provenance of the stellar IMF ?
In the present work we examined the hypothesis that, a core mass function
(CMF), such as the one deduced for cores in the Orion molecular cloud (OMC),
could possibly be the primogenitor of the stellar initial mass function (IMF).
Using the rate of accretion of a protostar from its natal core as a free
parameter, we demonstrate its quintessential role in determining the shape of
the IMF. By varying the rate of accretion, we show that a stellar mass
distribution similar to the universal IMF could possibly be generated starting
from either a typical CMF such as the one for the OMC, or a uniform
distribution of prestellar core masses which leads us to suggest, the apparent
similarity in shapes of the CMF and the IMF is perhaps, only incidental. The
apodosis of the argument being, complex physical processes leading to stellar
birth are crucial in determining the final stellar masses, and consequently,
the shape of stellar mass distribution. This work entails partial Monte-Carlo
treatment of the problem, and starting with a randomly picked sample of cores,
and on the basis of classical arguments which include protostellar feedback and
cooling due to emission from warm dust, a theoretical distribution of stellar
masses is derived for five realisations of the problem; the magnetic field,
though, has been left out of this exercise.Comment: 26 pages, 6 figures; To appear in New Astronom
- …