358,253 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
Young Low-Mass Stars and Brown Dwarfs in IC 348
I present new results from a continuing program to identify and characterize
the low-mass stellar and substellar populations in the young cluster IC 348
(1-10~Myr). Optical spectroscopy has revealed young objects with spectral types
as late as M8.25. The intrinsic J-H and H-K colors of these sources are
dwarf-like, whereas the R-I and I-J colors appear intermediate between the
colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are
reproduced well with averages of standard dwarf and giant spectra, suggesting
that such averages should be used in the classification of young late-type
sources. An H-R diagram is constructed for the low-mass population in IC 348
(K6-M8). The presumably coeval components of the young quadruple system GG~Tau
(White et al.) and the locus of stars in IC 348 are used as empirical
isochrones to test the theoretical evolutionary models. For the models of
Baraffe et al., an adjustment of the temperature scale to progressively warmer
temperatures at later M types, intermediate between dwarfs and giants, brings
all components of GG~Tau onto the same model isochrone and gives the population
of IC 348 a constant age and age spread as a function of mass. When other
observational constraints are considered, such as the dynamical masses of
GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most
consistent with observations of young systems. With compatible temperature
scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest
that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus,
several likely brown dwarfs are discovered in this study of IC 348, with masses
down to ~20-30 M_J.Comment: 23 pages, 9 figures, accepted to Ap
An empirical temperature calibration for the Delta a photometric system. II. The A-type and mid F-type star
With the Delta a photometric system, it is possible to study very distant
galactic and even extragalactic clusters with a high level of accuracy. This
can be done with a classical color-magnitude diagram and appropriate
isochrones. The new calibration presented in this paper is a powerful
extension. For open clusters, the reddening is straightforward for an
estimation via Isochrone fitting and is needed in order to calculate the
reddening-free, temperature sensitive, index (g1-y)0. As a last step, the
calibration can be applied to individual stars. Because no a-priori
reddening-free photometric parameters are available for the investigated
spectral range, we have applied the dereddening calibrations of the Stromgren
uvbybeta system and compared them with extinction models for the Milky Way. As
expected from the sample of bright stars, the extinction is negligible for
almost all objects. As a next step, already established calibrations within the
Stromgren uvbybeta, Geneva 7-color, and Johnson UBV systems were applied to a
sample of 282 normal stars to derive a polynomial fit of the third degree for
the averaged effective temperatures to the individual (g1-y)0 values with a
mean of the error for the whole sample of Delta T(eff) is 134K, which is lower
than the value in Paper I for hotter stars. No statistically significant effect
of the rotational velocity on the precision of the calibration was found.Comment: 5 pages, 2 figures, accepted by A&
Super-Eddington accretion rates in Narrow Line Seyfert 1 galaxies
Using the BH masses deduced from the empirical relation of Kaspi et al.
(2000) and assuming that the optical luminosity is provided by the accretion
disc, we show that Narrow Line Seyfert Galaxies 1 (NLS1s) accrete at
super-Eddington rates, while their luminosity stays of the order of the
Eddington limit. We take into account the possibility of a non-viscous energy
release in the gravitationally unstable region of the disc. It leads to a
smaller accretion rate and to a redder continuum than a standard disc, which
agrees better with the observations. The observed bolometric luminosities
appear to saturate at a few times the Eddington luminosity for super-Eddington
accretion rates, as predicted by slim disc models. The accretion rate stays
always of the order of a few M/yr, indicating that the growing of the
BH is mass supply limited . Since the masses of the BH increases by one order
of magnitude in a few 10 years, it could explain why NLS1s appear to not
follow the same BH - bulge relation as other galaxies. NLS1s should thus play
an important role in shaping the mass function of local BHs. We discuss the
possibility that the masses could be systematically underestimated due to an
inclination effect, and we conclude that the accretion rates could thus be
strongly overestimated, but only in a small proportion of objects.Comment: 13 pages, 8 figures, accepted in A &
- …