1,314 research outputs found
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
High-velocity runaway stars from three-body encounters
We performed numerical simulations of dynamical encounters between hard
massive binaries and a very massive star (VMS; formed through runaway mergers
of ordinary stars in the dense core of a young massive star cluster), in order
to explore the hypothesis that this dynamical process could be responsible for
the origin of high-velocity (\geq 200-400 km/s) early or late B-type stars. We
estimated the typical velocities produced in encounters between very tight
massive binaries and VMSs (of mass of \geq 200 Msun) and found that about 3-4
per cent of all encounters produce velocities of \geq 400 km/s, while in about
2 per cent of encounters the escapers attain velocities exceeding the Milky
Ways's escape velocity. We therefore argue that the origin of high-velocity
(\geq 200-400 km/s) runaway stars and at least some so-called hypervelocity
stars could be associated with dynamical encounters between the tightest
massive binaries and VMSs formed in the cores of star clusters. We also
simulated dynamical encounters between tight massive binaries and single
ordinary 50-100 Msun stars. We found that from 1 to \simeq 4 per cent of these
encounters can produce runaway stars with velocities of \geq 300-400 km/s
(typical of the bound population of high-velocity halo B-type stars) and
occasionally (in less than 1 per cent of encounters) produce hypervelocity
(\geq 700 km/s) late B-type escapers.Comment: 4 pages, 2 figure, to appear in Star Clusters -- Basic Galactic
Building Blocks throughout Time and Space, Proceed. of the IAU Symp. 266,
eds. R. de Grijs and J. Lepin
Geochronological database and classification system for age uncertainties in Neotropical pollen records.
The newly updated inventory of palaeoecological research in Latin America offers an important overview of sites available for multi-proxy and multi-site purposes. From the collected literature supporting this inventory, we collected all available age model metadata to create a chronological database of 5116 control points (e.g. 14C, tephra, fission track, OSL, 210Pb) from 1097 pollen records. Based on this literature review, we present a summary of chronological dating and reporting in the Neotropics. Difficulties and recommendations for chronology reporting are discussed. Furthermore, for 234 pollen records in northwest South America, a classification system for age uncertainties is implemented based on chronologies generated with updated calibration curves. With these outcomes age models are produced for those sites without an existing chronology, alternative age models are provided for researchers interested in comparing the effects of different calibration curves and age-depth modelling software, and the importance of uncertainty assessments of chronologies is highlighted. Sample resolution and temporal uncertainty of ages are discussed for different time windows, focusing on events relevant for research on centennial- to millennial-scale climate variability. All age models and developed R scripts are publicly available through figshare, including a manual to use the scripts
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
The spatial distribution of O-B5 stars in the solar neighborhood as measured by Hipparcos
We have developed a method to calculate the fundamental parameters of the
vertical structure of the Galaxy in the solar neighborhood from trigonometric
parallaxes alone. The method takes into account Lutz-Kelker-type biases in a
self-consistent way and has been applied to a sample of O-B5 stars obtained
from the Hipparcos catalog. We find that the Sun is located 24.2 +/- 1.7
(random) +/- 0.4 (systematic) pc above the galactic plane and that the disk
O-B5 stellar population is distributed with a scale height of 34.2 +/- 0.8
(random) +/- 2.5 (systematic) pc and an integrated surface density of (1.62 +/-
0.04 (random) +/- 0.14 (systematic)) 10^{-3} stars pc^{-2}. A halo component is
also detected in the distribution and constitutes at least ~5% of the total
O-B5 population. The O-B5 stellar population within ~100 pc of the Sun has an
anomalous spatial distribution, with a less-than-average number density. This
local disturbance is probably associated with the expansion of Gould's belt.Comment: 14 pages, 3 figures, to appear in the May 2001 issue of the
Astronomical Journa
Hypervelocity Stars. I. The Spectroscopic Survey
We discuss our targeted search for hypervelocity stars (HVSs), stars
traveling with velocities so extreme that dynamical ejection from a massive
black hole is their only suggested origin. Our survey, now half complete, has
successfully identified a total of four probable HVSs plus a number of other
unusual objects. Here we report the most recently discovered two HVSs: SDSS
J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with
Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s,
respectively. The other late B-type objects in our survey are consistent with a
population of post main-sequence stars or blue stragglers in the Galactic halo,
with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s.
Interestingly, the velocity distribution shows a tail of objects with large
positive velocities that may be a mix of low-velocity HVSs and high-velocity
runaway stars. Our survey also includes a number of DA white dwarfs with
unusually red colors, possibly extremely low mass objects. Two of our objects
are B supergiants in the Leo A dwarf, providing the first spectroscopic
evidence for star formation in this dwarf galaxy within the last ~30 Myr.Comment: 10 pages, uses emulateapj, accepted by Ap
Young Globular Clusters and Dwarf Spheroidals
Most of the globular clusters in the main body of the Galactic halo were
formed almost simultaneously. However, globular cluster formation in dwarf
spheroidal galaxies appears to have extended over a significant fraction of a
Hubble time. This suggests that the factors which suppressed late-time
formation of globulars in the main body of the Galactic halo were not operative
in dwarf spheroidal galaxies. Possibly the presence of significant numbers of
``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption
that many of these objects were formed in Sagittarius-like (but not
Fornax-like) dwarf spheroidal galaxies, that were subsequently destroyed by
Galactic tidal forces. It would be of interest to search for low-luminosity
remnants of parental dwarf spheroidals around the ``young'' globulars Eridanus,
Palomar 1, 3, 14, and Terzan 7. Furthermore multi-color photometry could be
used to search for the remnants of the super-associations, within which outer
halo globular clusters originally formed. Such envelopes are expected to have
been tidally stripped from globulars in the inner halo.Comment: 18 pages, with 2 figures, in LaTeX format; to appear in the
Astrophysical Journal in February 200
Local Pulsars; A note on the Birth-Velocity Distribution
We explore a simple model for the representation of the observed
distributions of the motions, and the characteristic ages of the local
population of pulsars. The principal difference from earlier models is the
introduction of a unique value, S, for the kick velocity with which pulsars are
born. We consider separately the proper motion components in galactic longitude
and latitude, and find that the distributions of the velocity components
parallel and perpendicular to the galactic plane are represented satisfactorily
by S=200 km/sec, and leave no room for a significant fraction of much higher
velocities. The successful proposition of a unique value for the kick velocity
may provide an interesting tool in attempts to understand the physical process
leading to the expulsion of the neutron star.Comment: To be published in JAA, 14 pages, 7 figure
- …