312 research outputs found
The early evolution of Globular Clusters: the case of NGC 2808
Enhancement and spread of helium among globular cluster stars have been
recently suggested as a way to explain the horizontal branch blue tails, in
those clusters which show a primordial spread in the abundances of CNO and
other elements involved in advanced CNO burning (D'Antona et al. 2002). In this
paper we examine the implications of the hypothesis that, in many globular
clusters, stars were born in two separate events: an initial burst (first
generation), which gives origin to probably all high and intermediate mass
stars and to a fraction of the cluster stars observed today, and a second,
prolonged star formation phase (second generation) in which stars form directly
from the ejecta of the intermediate mass stars of the first generation. In
particular, we consider in detail the morphology of the horizontal branch in
NGC 2808 and argue that it unveils the early cluster evolution, from the birth
of the first star generation to the end of the second phase of star formation.
This framework provides a feasible interpretation for the still unexplained
dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the
RR Lyr region to the gap in the helium content between the red clump, whose
stars are considered to belong to the first stellar generation and have
primordial helium, and the blue side of the horizontal branch, whose minimum
helium content reflects the helium abundance in the smallest mass
(~4Msun)contributing to the second stellar generation. This scenario provides
constraints on the required Initial Mass Function, in a way that a great deal
of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa
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
Lithium Depletion Boundary in a Pre-Main Sequence Binary System
A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a
\~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A
absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature
is detected from the primary (~M4). The physical companionship of the two stars
is confirmed from common proper motions. Current theoretical pre-main sequence
evolutionary models cannot simultaneously match the observed colors,
brightnesses, and Li depletion patterns of this binary system. At the age upper
limit of 20 Myr, contemporary theoretical evolutionary models predict too slow
Li depletion. If true Li depletion is a faster process than predicted by
theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391)
estimated from the Li depletion boundary method are all overestimated. Because
of the importance of the open cluster age scale, development of self-consistent
theoretical models to match the HIP 112312 data is desirable.Comment: Accepted in ApJL. 5 pages total (3 tables, 3 figures
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster
We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)
and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most
massive star in the Orion Nebula Cluster (ONC). At the age and distance of this
cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo)
occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our
photometry is complete for source detection at the 7 sigma level to K~17.5 mag
and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual
extinction values as high as 10 magnitudes. We use the observed magnitudes,
colors, and star counts to constrain the shape of the inner ONC stellar mass
function across the hydrogen burning limit. After determining the stellar age
and near-infrared excess properties of the optically visible stars in this same
inner ONC region, we present a new technique that incorporates these
distributions when extracting the mass function from the observed density of
stars in the K-(H-K) diagram. We find that our data are inconsistent with a
mass function that rises across the stellar/sub-stellar boundary. Instead, we
find that the most likely form of the inner ONC mass function is one that rises
to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit
with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply
to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC
as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal.
Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc
The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs
Using the largest high-resolution spectroscopic sample to date of young, very
low mass stars (VLMS) and brown dwarfs (BDs), we investigate disk accretion in
objects ranging from just above the hydrogen-burning limit all the way to
nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or
masses from 0.15 Msun down to ~15 Jupiters. They are confirmed members of the
rho Oph, Taurus, Cha I, IC 348, R CrA, Upper Sco and TW Hydrae regions, with
ages = M6.5). We
find that: (1) classical T Tauri-like disk-accretion persists in the BD domain
down to nearly the deuterium-burning limit; (2) in addition to H-alpha,
permitted emission lines of CaII, OI and HeI are also good accretion
indicators, as in CTTs; (3) the CaII 8662A flux is an excellent quantitative
measure of the accretion rate (Mdot) in VLMS and BDs(as in CTTs); (4) Mdot
diminishes as M^2 -- our measurements support previous findings of this
correlation, and extend it to the entire range of sub-stellar masses; (5) the
accretor fraction among VLMS and BDs decreases substantially with age, as in
higher-mass stars; (6) at any given age, the VLMS and BD accretor fraction is
comparable to that in higher-mass stars; and (7) a number of sources with IR
disk excesses do not evince measurable accretion, with the incidence of such a
mismatch increasing with age: this implies that disks in the low mass regime
can persist beyond the main accretion phase, and parallels the transition from
the classical to post-T Tauri stage in more massive stars. These strong
similarities at young ages, between higher-mass stars and low-mass bodies close
to and below the hydrogen-burning limit, are consistent with a common formation
mechanism in the two mass regimes. (abridged)Comment: 64 pages, 7 figures. ApJ accepte
A Test of Pre-Main-Sequence Lithium Depletion Models
Despite the extensive study of lithium depletion during pre-main-sequence
contraction, studies of individual stars show discrepancies between ages
determined from the HR diagram and ages determined from lithium depletion (Song
et al. 2002, White & Hillenbrand 2005) indicating open questions in the
pre-main-sequence evolutionary models. To further test these models, we present
high resolution spectra for members of the Beta Pictoris Moving Group (BPMG),
which is young and nearby. We measure equivalent widths of the 6707.8 Angstrom
Li I line in these stars and use them to determine lithium abundances. We
combine the lithium abundance with the predictions of pre-main-sequence
evolutionary models in order to calculate a lithium depletion age for each
star. We compare this age to the age predicted by the HR diagram of the same
model. We find that the evolutionary models under-predict the amount of lithium
depletion for the BPMG given its nominal HR diagram age of ~12 Myr (Zuckerman
et al. 2001), particularly for the mid-M stars, which have no observable Li I
line. This results in systematically older ages calculated from lithium
depletion isochrones than from the HR diagram. We suggest that this discrepancy
may be related to the discrepancy between measured M-dwarf radii and the
smaller radii predicted by evolutionary models.Comment: Accepted by ApJ; 21 pages, 5 figure
A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
The newly discovered brown-dwarf eclipsing binary 2MASS J05352184-0546085
provides a unique laboratory for testing the predictions of theoretical models
of brown-dwarf formation and evolution. The finding that the lower-mass brown
dwarf in this system is hotter than its higher-mass companion represents a
challenge to brown-dwarf evolutionary models, none of which predict this
behavior. Here we present updated determinations of the basic physical
properties of 2M0535-05, bolstering the surprising reversal of temperatures
with mass in this system. We compare these measurements with widely used
brown-dwarf evolutionary tracks, and find that the temperature reversal can be
explained by some models if the components of 2M0535-05 are mildly non-coeval,
possibly consistent with dynamical simulations of brown-dwarf formation.
Alternatively, a strong magnetic field on the higher-mass brown dwarf might
explain its anomalously low surface temperature, consistent with emerging
evidence that convection is suppressed in magnetically active, low-mass stars.
Finally, we discuss future observational and theoretical work needed to further
characterize and understand this benchmark system.Comment: 31 pages, 7 figures, accepted by Ap
PSR J1740-5340: accretion inhibited by radio-ejection in a binary millisecond pulsar in the Globular Cluster NGC 6397
We present an evolutionary scenario for the spin-up and evolution of binary
millisecond pulsars, according to which the companion of the pulsar PSR J
1740-5340, recently discovered as a binary with orbital period of 32.5 hr in
the Globular Cluster NGC 6397, is presently in a phase of ``radio-ejection''
mass loss from the system. At present, Roche lobe overflow due to the nuclear
evolution of the pulsar companion and to systemic angular momentum losses by
magnetic braking is still going on, but accretion is inhibited by the momentum
exerted by the radiation of the pulsar on the matter at the inner Lagrangian
point. The presence of this matter around the system is consistent with the
long lasting irregular radio eclipses seen in the system. Roche lobe
deformation of the mass losing component is also necessary to be compatible
with the optical light curve. The "radio-ejection" phase had been recently
postulated by us to deal with the problem of the lack of submillisecond pulsars
(Burderi et al. 2001, ApJ, 560, L71).Comment: Accepted for publication in The Astrophysical Journa
A Double Main Sequence in the Globular Cluster NGC 6397
High-precision multi-band HST photometry reveals that the main sequence (MS)
of the globular cluster NGC 6397 splits into two components, containing ~30%
and ~70% of the stars. This double sequence is consistent with the idea that
the cluster hosts two stellar populations: (i) a primordial population that has
a composition similar to field stars, and containing ~30% of the stars, and
(ii) a second generation with enhanced sodium and nitrogen, depleted carbon and
oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the
color difference between the two sequences across a variety of color baselines
and find that the second sequence is anomalously faint in m_F336W. Theoretical
isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap
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