1,541 research outputs found
Spectral Types of Field and Cluster O-Stars
The recent catalog of spectral types of Galactic O-type stars by
Mai'z-Apella'niz et al. has been used to study the differences between the
frequencies of various subtypes of O-type stars in the field, in OB
associations and among runaway stars. At a high level of statistical
significance the data show that O-stars in clusters and associations have
earlier types (and hence presumably larger masses and/or younger ages) than
those that are situated in the general field. Furthermore it is found that the
distribution of spectral subtypes among runaway O-stars is indistinguishable
from that among field stars, and differs significantly from that of the O-type
stars that are situated in clusters and associations. The difference is in the
sense that runaway O-stars, on average, have later subtypes than do those that
are still located in clusters and associations.Comment: To be published in the October 2004 issue of the Astronomical Journal
Included Figure 1, page
The origin of runaway stars
Milli-arcsecond astrometry provided by Hipparcos and by radio observations
makes it possible to retrace the orbits of some of the nearest runaway stars
and pulsars to determine their site of origin. The orbits of the runaways AE
Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each
other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these
runaways were formed in a binary-binary encounter. The path of the runaway star
zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr
ago, in the young stellar group Upper Scorpius. We propose that this neutron
star is the remnant of a supernova that occurred in a binary system which also
contained zeta Oph, and deduce that the pulsar received a kick velocity of
about 350 km/s in the explosion. These two cases provide the first specific
kinematic evidence that both mechanisms proposed for the production of runaway
stars, the dynamical ejection scenario and the binary-supernova scenario,
operate in nature.Comment: 5 pages, including 2 eps-figures and 1 table, submitted to the ApJ
Letters. The manuscript was typeset using aaste
Bayesian analysis of 210Pb dating
In many studies of environmental change of the past few centuries, 210Pb
dating is used to obtain chronologies for sedimentary sequences. One of the
most commonly used approaches to estimate the ages of depths in a sequence is
to assume a constant rate of supply (CRS) or influx of `unsupported' 210Pb from
the atmosphere, together with a constant or varying amount of `supported'
210Pb. Current 210Pb dating models do not use a proper statistical framework
and thus provide poor estimates of errors. Here we develop a new model for
210Pb dating, where both ages and values of supported and unsupported 210Pb
form part of the parameters. We apply our model to a case study from Canada as
well as to some simulated examples. Our model can extend beyond the current CRS
approach, deal with asymmetric errors and mix 210Pb with other types of dating,
thus obtaining more robust, realistic and statistically better defined
estimates.Comment: 22 Pages, 4 Figure
A Hipparcos census of the nearby OB associations
A comprehensive census of the stellar content of the nearby OB associations
is presented, based on Hipparcos positions, proper motions, and parallaxes.
Moving groups are identified by combining de Bruijne's refurbished convergent
point method with the `Spaghetti method' of Hoogerwerf & Aguilar. Monte Carlo
simulations are used to estimate the expected number of interloper field stars.
Astrometric members are listed for 12 young stellar groups, out to a distance
of ~650 pc. These are the 3 subgroups Upper Scorpius, Upper Centaurus Lupus and
Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2,
alpha Persei (Per OB3), Cas-Tau, Lac OB1, Cep OB2, and a new group designated
as Cep OB6. The selection procedure corrects the list of previously known
astrometric and photometric B- and A-type members, and identifies many new
members, including a significant number of F stars, as well as evolved stars,
e.g., the Wolf-Rayet stars gamma^2 Vel (Vel OB2) and EZ CMa (Col 121), and the
classical Cepheid delta Cep in Cep OB6. In the nearest associations the
later-type members include T Tauri objects and other pre-main sequence stars.
Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon
OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is
inconclusive, due to their large distance or unfavorable kinematics.
The mean distances of the well-established groups are systematically smaller
than previous estimates. The mean motions display a systematic pattern, which
is discussed in relation to the Gould Belt. Six of the 12 detected moving
groups do not appear in the classical list of nearby OB associations. The
number of unbound young stellar groups in the Solar neighbourhood may be
significantly larger than thought previously.Comment: 51 pages, 30 PostScript figures, 6 tables in PostScript format,
default LaTeX using psfig.sty; accepted for publication in the Astronomical
Journal, scheduled for January 1999 issue. Abbreviated abstrac
OB Associations
Since the previous (1990) edition of this meeting enormous progress in the
field of OB associations has been made. Data from X-ray satellites have greatly
advanced the study of the low-mass stellar content of associations, while
astrometric data from the Hipparcos satellite allow for a characterization of
the higher-mass content of associations with unprecedented accuracy. We review
recent work on the OB associations located within 1.5 kpc from the Sun, discuss
the Hipparcos results at length, and point out directions for future research.Comment: To appear in The Physics of Star Formation and Early Stellar
Evolution II, eds C.J. Lada & N. Kylafis (Kluwer Academic), 30 pages, 9
EPS-figures, LaTeX using crckapb.sty, epsfig.sty, amssymb.st
Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars
Evidence from pulsar wind nebula symmetry axes and radio polarization
observations suggests that pulsar motions correlate with the spin directions.
We assemble this evidence for young isolated pulsars and show how it can be
used to quantitatively constrain birth kick scenarios. We illustrate by
computing several plausible, but idealized, models where the momentum thrust is
proportional to the neutrino cooling luminosity of the proto-neutron star. Our
kick simulations include the effects of pulsar acceleration and spin-up and our
maximum likelihood comparison with the data constrains the model parameters.
The fit to the pulsar spin and velocity measurements suggests that: i) the
anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a
pre-kick spin of the star is required, the preferred magnitude is small
10-20rad/s, so that for the best-fit models iii) the bulk of the spin is
kick-induced with ~120rad/s and iv) the models suggest that the
anisotropy emerges on a timescale ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte
The Becklin-Neugebauer Object as a Runaway B Star, Ejected 4000 years ago from the theta^1C system
We attempt to explain the properties of the Becklin-Neugebauer (BN) object as
a runaway B star, as originally proposed by Plambeck et al. (1995). This is one
of the best-studied bright infrared sources, located in the Orion Nebula
Cluster -- an important testing ground for massive star formation theories.
From radio observations of BN's proper motion, we trace its trajectory back
to Trapezium star theta^1C, the most massive (45 Msun) in the cluster and a
relatively tight (17 AU) visual binary with a B star secondary. This origin
would be the most recent known runaway B star ejection event, occurring only
\~4000 yr ago and providing a unique test of models of ejection from multiple
systems of massive stars. Although highly obscured, we can constrain BN's mass
(~7 Msun) from both its bolometric luminosity and the recoil of theta^1C.
Interaction of a runaway B star with dense ambient gas should produce a compact
wind bow shock. We suggest that X-ray emission from this shocked gas may have
been seen by Chandra: the offset from the radio position is ~300 AU in the
direction of BN's motion. Given this model, we constrain the ambient density,
wind mass-loss rate and wind velocity. BN made closest approach to the massive
protostar, source ``I'', 500 yr ago. This may have triggered enhanced accretion
and thus outflow, consistent with previous interpretations of the outflow being
a recent (~10^3 yr) "explosive" event.Comment: 6 pages, accepted to ApJ Letter
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