653 research outputs found
IP Eri: A surprising long-period binary system hosting a He white dwarf
We determine the orbital elements for the K0 IV + white dwarf (WD) system IP
Eri, which appears to have a surprisingly long period of 1071 d and a
significant eccentricity of 0.25. Previous spectroscopic analyses of the WD,
based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a
mass of only 0.43 M, implying it to be a helium-core WD. The orbital
properties of IP Eri are similar to those of the newly discovered long-period
subdwarf B star (sdB) binaries, which involve stars with He-burning cores
surrounded by extremely thin H envelopes, and are therefore close relatives to
He WDs. We performed a spectroscopic analysis of high-resolution spectra from
the HERMES/Mercator spectrograph and concluded that the atmospheric parameters
of the K0 component are K, , [Fe/H] = 0.09
and km/s. The detailed abundance analysis focuses on C, N, O
abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process
(Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree
with those of normal field stars of the same metallicity. The long period and
non-null eccentricity indicate that this system cannot be the end product of a
common-envelope phase; it calls instead for another less catastrophic
binary-evolution channel presented in detail in a companion paper (Siess et al.
2014).Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&A
(Update of Table 3, Fig. 8 and text in Sect. 5.1, 5.3 and 6 due to minor
corrections on N and Y II
CNO enrichment by rotating AGB stars in globular clusters
AGB stars have long been held responsible for the important star-to-star
variations in light elements observed in Galactic globular clusters. We analyse
the main impacts of a first generation of rotating intermediate-mass stars on
the chemical properties of second-generation globular cluster stars. The
rotating models were computed without magnetic fields and without the effects
of internal gravity waves. They account for the transports by meridional
currents and turbulence. We computed the evolution of both standard and
rotating stellar models with initial masses between 2.5 and 8 Msun within the
metallicity range covered by Galactic globular clusters. During central
He-burning, rotational mixing transports fresh CO-rich material from the core
towards the hydrogen-burning shell, leading to the production of primary 14N.
In stars more massive than M > 4 Msun, the convective envelope reaches this
reservoir during the second dredge-up episode, resulting in a large increase in
the total C+N+O content at the stellar surface and in the stellar wind. The
corresponding pollution depends on the initial metallicity. At low- and
intermediate-metallicity, it is at odds with the constancy of C+N+O observed
among globular cluster low-mass stars. With the given input physics, our models
suggest that massive rotating AGB stars have not shaped the abundance patterns
observed in low- and intermediate-metallicity globular clusters. Our
non-rotating models, on the other hands, do not predict surface C+N+O
enhancements, hence are in a better position as sources of the chemical
anomalies in globular clusters showing the constancy of the C+N+O. However at
the moment, there is no reason to think that intermediate mass stars were not
rotating.Comment: Accepted in Astronomy and Astrophysics, 7 pages, 3 figure
The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high resolution near-infrared spectroscopy with the Keck telescope, we
have detected the radial velocity signatures of the cool secondary components
in four optically identified pre-main-sequence, single-lined spectroscopic
binaries. All are weak-lined T Tauri stars with well-defined center of mass
velocities. The mass ratio for one young binary, NTTS 160905-1859, is M2/M1 =
0.18+/-0.01, the smallest yet measured dynamically for a pre-main-sequence
spectroscopic binary. These new results demonstrate the power of infrared
spectroscopy for the dynamical identification of cool secondaries. Visible
light spectroscopy, to date, has not revealed any pre-main-sequence secondary
stars with masses <0.5 M_sun, while two of the young systems reported here are
in that range. We compare our targets with a compilation of the published young
double-lined spectroscopic binaries and discuss our unique contribution to this
sample.Comment: Accepted for publication in the April, 2002, ApJ; 6 figure
Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy
Near IR spectra obtained with ISAAC at VLT, have been used to pose
constraints on the evolutionary state and accretion properties of a sample of
five embedded YSOs located in the R CrA core. This sample includes three Class
I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6
and IRS3). Absorption lines have been detected in the medium resolution spectra
of all the observed targets, together with emission lines likely originating in
the disk-star-wind connected regions. We derived spectral types, veiling and
stellar luminosity of the five observed sources, which in turn have been used
to infer their mass and age adopting pre-main sequence evolutionary tracks. We
find that in HH100 IR and IRS2 most of the bolometric luminosity is due to
accretion, while the other three investigated sources, including the Class I
object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We
observe a general correlation between the accretion luminosity, the IR veiling
and the emission line activity of the sources. A correlation between the
accretion activity and the spectral energy distribution slope is recognizable
but with the notable exception of IRS5a. Our analysis therefore shows how the
definition of the evolutionary stage of deeply embedded YSOs by means of IR
colors needs to be more carefully refined.Comment: 17 pages, 13 figures, accepted on A&
From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory and main
sequence stellar evolution theory to accurately determine stellar population
ages by comparing ages derived using both techniques for open clusters ranging
from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence
evolutionary ages over the entire age range currently available for study. I
also find that directly comparing main sequence turn-off ages to white dwarf
ages is only weakly sensitive to realistic levels of errors in cluster
distance, metallicity, and reddening. Additional detailed comparisons between
white dwarf and main sequence ages have tremendous potential to refine and
calibrate both of these important clocks, and I present new simulations of
promising open cluster targets. The most demanding requirement for these white
dwarf studies are very deep (V > 25-28) cluster observations made necessary by
the faintness of the oldest white dwarfs.Comment: 25 pages, incl. 10 figures, ApJ accepted for April, 200
Turbulent Flow-Driven Molecular Cloud Formation: A Solution to the Post-T Tauri Problem?
We suggest that molecular clouds can be formed on short time scales by
compressions from large scale streams in the interstellar medium (ISM). In
particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc
2-5 pc, most have been formed by H I flows in Myr,
explaining the absence of post-T Tauri stars in the region with ages Myr. Observations in the 21 cm line of the H I `halos' around the Taurus
molecular gas show many features (broad asymmetric profiles, velocity shifts of
H I relative to CO) predicted by our MHD numerical simulations, in which
large-scale H I streams collide to produce dense filamentary structures. This
rapid evolution is possible because the H I flows producing and disrupting the
cloud have much higher velocities (5-10 kms) than present in the molecular gas
resulting from the colliding flows. The simulations suggest that such flows can
occur from the global ISM turbulence without requiring a single triggering
event such as a SN explosion.Comment: 26 pages, 12 ps figures. Apj accepte
Spitzer observations of the Orion OB1 association: disk census in the low mass stars
We present new Spitzer Space Telescope observations of two fields in the
Orion OB1 association. We report here IRAC/MIPS observations for 115 confirmed
members and 41 photometric candidates of the ~10 Myr 25 Orionis aggregate in
the OB1a subassociation, and 106 confirmed members and 65 photometric
candidates of the 5 Myr region located in the OB1b subassociation. The 25
Orionis aggregate shows a disk frequency of 6% while the field in the OB1b
subassociation shows a disk frequency of 13%. Combining IRAC, MIPS and 2MASS
photometry we place stars bearing disks in several classes: stars with
optically thick disks (class II systems), stars with an inner transitional
disks (transitional disk candidates) and stars with "evolved disks"; the last
exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify 1
transitional disk candidate in the 25 Orionis aggregate and 3 in the OB1b
field; this represents ~10% of the disk bearing stars, indicating that the
transitional disk phase can be relatively fast. We find that the frequency of
disks is a function of the stellar mass, suggesting a maximum around stars with
spectral type M0. Comparing the infrared excess in the IRAC bands among several
stellar groups we find that inner disk emission decays with stellar age,
showing a correlation with the respective disk frequencies. The disk emission
at the IRAC and MIPS bands in several stellar groups indicates that disk
dissipation takes place faster in the inner region of the disks. Comparison
with models of irradiated accretion disks, computed with several degrees of
settling, suggests that the decrease in the overall accretion rate observed in
young stellar groups is not sufficient to explain the weak disk emission
observed in the IRAC bands for disk bearing stars with ages 5 Myr or older.Comment: Accepted in the Astrophysical Journa
Debris Disks in NGC 2547
We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks
using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to
mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only
two to four stars with excesses at 8 um out of ~400-500 cluster members,
resulting in an excess fraction <~1 percent at this wavelength. By contrast,
the excess fraction at 24 um is ~40 percent (for B-F types). Out of four
late-type stars with excesses at 8 um two marginal ones are consistent with
asteroid-like debris disks. Among stars with strong 8 um excesses one is
possibly from a transitional disk, while another one can be a result of a
catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts
of disks around solar-type stars clear out very thoroughly by 30 Myrs of age.
Comparing with the much slower decay of excesses at 24 and 70 um, disks clear
from the inside out, of order 10 Myr for the inner zones probed at 8 um
compared with a hundred or more Myr for those probed with the two longer
wavelengths.Comment: Accepted to ApJ, 29 pages, 13 figs. A Note in Proof concerning
cluster's age was added in the original submission of 2007 July 19. Full
Tables 1 and 2 in the electronic form together with the article with full
resolution figures are available at
http://www.astro.ufl.edu/~ngorlova/disksNGC2547
Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars
Among Carbon-Enhanced Metal-Poor (CEMP) stars, some are found to be enriched
in s-process elements (CEMP-s), in r-process elements (CEMP-r) or in both s-
and r-process elements (CEMP-rs). The origin of the abundance differences
between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that
the i-process, whose site still remains to be identified, could better
reproduce CEMP-rs abundances than the s-process. We analyze high-resolution
spectra of 25 metal-poor stars, observed with the high-resolution HERMES
spectrograph mounted on the Mercator telescope, La Palma, or with the UVES/VLT
and HIRES/KECK spectrographs. We propose a new, robust classification method
for CEMP-s and CEMP-rs stars using eight heavy element abundances. The
abundance profiles of CEMP-s and CEMP-rs stars are derived and there appears to
be an abundance continuum between the two stellar classes. CEMP-rs stars
present most of the characteristics of extrinsic stars such as CEMP-s, CH,
Barium and extrinsic S stars, with an even larger binarity rate among CEMP-rs
stars than among CEMP-s stars. Stellar evolutionary tracks of an enhanced
carbon composition (consistent with our abundance determinations) are necessary
to explain the position of CEMP-s and CEMP-rs stars in the HR diagram using
Gaia DR2 parallaxes; they are found to lie mostly on the RGB. CEMP-rs stars can
be explained as being polluted by a low-mass, low-metallicity TP-AGB companion
experiencing i-process nucleosynthesis after proton ingestion during its first
convective thermal pulses. The global fitting of our i-process models to
CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. As
such, CEMP-rs stars could be renamed as CEMP-sr stars, since they represent a
particular manifestation of the s-process at low-metallicities. For these
objects a call for an exotic i-process site may not necessarily be required
anymore.Comment: 35 pages, 26 figures, Accepted for publication in A&A, Final versio
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
- âŠ