1,034 research outputs found
Hydrogenic Transitions in Multiply Charged Fe and Ni Ions
Ten lines in the range 3880≦λ≦5666 Å in the beam-foil spectrum of iron have been identified with specific hydrogenic transitions in Fe iv-viii. The same transitions were observed from Ni and Ar beams. Deviations from the hydrogenic wavelengths are shown to be consistent with that expected from core polarization. The absence of these lines in astrophysical sources is discussed. A wavelength table is presented for identification of hydrogenic transitions to be expected in beam-foil spectra
Simulations of Stellar Collisions Involving Pre-Main Sequence Stars
In this paper, we present the results of smoothed particle hydrodynamic (SPH)
simulations of collisions between pre-main sequence stars and a variety of
other kinds of stars. Simulations over a range of impact parameters and
velocities were performed. We find that pre-main sequence stars tend to ``wrap
themselves'' around their impactor. We discuss the probable evolutionary state
of products of collisions between pre-main sequence stars and pre-main
sequence, main sequence, giant branch, and compact stars. The nature of the
collision product does not depend strongly on the impact parameter or the
velocity of the collision.Comment: Accepted by Ap
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Intrauterine pregnancy following low-dose gonadotropin ovulation induction and direct intraperitoneal insemination for severe cervical stenosis
BACKGROUND: We present a case of primary infertility related to extreme cervical stenosis, a subset of cervical factor infertility which accounts for approximately 5% of all clinical infertility referrals. CASE PRESENTATION: A 37 year-old nulligravida was successfully treated with ovulation induction via recombinant follicle stimulating hormone (FSH) and direct intraperitoneal insemination (IPI). Anticipating controlled ovarian hyperstimulation with in vitro fertilization/embryo transfer (IVF), the patient underwent hysteroscopy and cervical recanalization, but safe intrauterine access was not possible due to severe proximal cervical stricture. Hysterosalpingogram established bilateral tubal patency and confirmed an irregular cervical contour. Since the cervical canal could not be traversed, neither standard intrauterine insemination nor transcervical embryo transfer could be offered. Prepared spermatozoa were therefore placed intraperitoneally at both tubal fimbria under real-time transvaginal sonographic guidance using a 17 gage single-lumen IVF needle. Supplementary progesterone was administered as 200 mg/d lozenge (troche) plus 200 mg/d rectal suppository, maintained from the day following IPI to the 8(th )gestational week. A singleton intrauterine pregnancy was achieved after the second ovulation induction attempt. CONCLUSIONS: In this report, we outline the relevance of cervical factor infertility to reproductive medicine practice. Additionally, our andrology evaluation, ovulation induction approach, spermatozoa preparation, and insemination technique in such cases are described
Discovery of another peculiar radial distribution of Blue Stragglers in Globular Clusters: The case of 47 Tuc
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) in the
globular cluster 47 Tuc spanning the entire radial extent of the cluster.
The BSS distribution is highly peaked in the cluster center, rapidly
decreases at intermediate radii, and finally rises again at larger radii. The
observed distribution closely resembles that discovered in M3 by Ferraro et al
(1993,1997). To date, complete BSS surveys covering the full radial extent (HST
in the center and wide field CCD ground based observations of the exterior)
have been performed for only these two clusters. Both show a bimodal radial
distribution, despite their different dynamical properties. BSS surveys
covering the full spatial extent of more globular clusters are clearly required
to determine how common bimodality is and what its consequence is for theories
of BSS formation and cluster dynamics.Comment: 21 pages, 6 figures, accepted for pubblication in Ap
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Modest-2: A Summary
This is a summary paper of MODEST-2, a workshop held at the Astronomical
Institute ``Anton Pannekoek'' in Amsterdam, 16-17 December 2002. MODEST is a
loose collaboration of people interested in MOdelling DEnse STellar systems,
particularly those interested in modelling these systems using all the
available physics (stellar dynamics, stellar evolution, hydrodynamics and the
interplay between the three) by defining interfaces between different codes. In
this paper, we summarize 1) the main advances in this endeavour since MODEST-1;
2) the main science goals which can be and should be addressed by these types
of simulations; and 3) the most pressing theoretical and modelling advances
that we identified.Comment: Accepted by New Astronom
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
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