44 research outputs found
Kinematic Control of the Inertiality of the System of Tycho-2 and UCAC2 Stellar Proper Motions
Based on the Ogorodnikov-Milne model, we analyze the proper motions of
Tycho-2 and UCAC2 stars. We have established that the model component that
describes the rotation of all stars under consideration around the Galactic y
axis differs significantly from zero at various magnitudes. We interpret this
rotation found using the most distant stars as a residual rotation of the
ICRS/Tycho-2 system relative to the inertial reference frame. For the most
distant ( pc) Tycho-2 and UCAC2 stars, the mean rotation around
the Galactic y axis has been found to be mas yr.
The proper motions of UCAC2 stars with magnitudes in the range are
shown to be distorted appreciably by the magnitude equation in
, which has the strongest effect for northern-sky stars
with a coefficient of mas yr mag. We have detected
no significant effect of the magnitude equation in the proper motions of UCAC2
stars brighter than .Comment: 15 pages, 6 figure
Phase models of the Milky Way stellar disc
We present a new iterative method for constructing equilibrium phase models
of stellar systems. Importantly, this method can provide phase models with
arbitrary mass distributions. The method is based on the following principle.
Our task is to generate an equilibrium N-body system with a given mass
distribution. For this purpose, we let the system reach equilibrium through its
dynamical evolution. During this evolution we hold mass distribution in this
system. This principle is realized in our method by means of an iterative
procedure. We have used our method to construct a phase model of the disc of
our Galaxy. In our method, we use the mass distribution in the Galaxy as input
data. Here we used two Galactic density models (suggested by Flynn,
Sommer-Larsen and Christensen and by Dehnen and Binney). For a fixed-mass model
of the Galaxy we can construct a one-parameter family of equilibrium models of
the Galactic disc. We can, however, choose a unique model using local kinematic
parameters that are known from Hipparcos data. We show that the phase models
constructed using our method are close to equilibrium. The problem of
uniqueness for our models is discussed, and we discuss some further
applications of our method.Comment: 16 pages, 10 figure
Galactic Rotation Parameters from Data on Open Star Clusters
Currently available data on the field of velocities Vr, Vl, Vb for open star
clusters are used to perform a kinematic analysis of various samples that
differ by heliocentric distance, age, and membership in individual structures
(the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters
located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the
Galactic rotation parameters
Wo =-26.0+-0.3 km/s/kpc,
W'o = 4.18+-0.17 km/s/kpc^2,
W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1
km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo =
0+-1 degrees. The Galactocentric distance Ro in the model used has been found
to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7
kpc and 5.6+-0.3 kpc for the samples of young (50 Myr)
clusters, respectively. Our study of the kinematics of young open star clusters
in various spiral arms has shown that the kinematic parameters are similar to
the parameters obtained from the entire sample for the Carina-Sagittarius and
Perseus arms and differ significantly from them for the Orion arm. The
contraction effect is shown to be typical of star clusters with various ages.
It is most pronounced for clusters with a mean age of 100 Myr, with the
contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
The Parker Instability in 3-D: Corrugations and Superclouds Along the Carina-Sagittarius Arm
Here we present three-dimensional MHD models for the Parker instability in a
thick magnetized disk, including the presence of a spiral arm. The -field is
assumed parallel to the arm, and the model results are applied to the optical
segment of the Carina-Sagittarius arm. The characteristic features of the
undular and interchange modes are clearly apparent in the simulations. The
undular mode creates large gas concentrations distributed along the arm. This
results in a clear arm/inter-arm difference: the instability triggers the
formation of large interstellar clouds inside the arms, but generates only
small structures with slight density enhancements in the inter-arm regions. The
resulting clouds are distributed in an antisymmetric way with respect to the
midplane, creating an azimuthal corrugation along the arm. For conditions
similar to those of the optical segment of the Carina-Sagittarius arm, it has a
wavelength of about 2.4 kpc. This structuring can explain the origin of both HI
superclouds and the azimuthal corrugations in spiral arms. The wavelength
matches the corrugation length derived with the young stellar groups located in
the optical segment of the Carina-Sagittarius arm. Keywords: Galaxy: kinematics
and dynamics -- Galaxy: structure -- Instabilities -- ISM: clouds -- ISM:
magnetic fields -- ISM: structure -- MHDComment: 29 pages, 12 figures, Latex, Accepted by the Astrophysical Journa
New Studies of the Pulsar Wind Nebula in the Supernova Remnant CTB 80
We investigated the kinematics of the pulsar wind nebula (PWN) associated
with PSR B1951+32 in the old supernova remnant CTB 80 using the Fabry-Perot
interferometer of the 6m Special Astrophysical Observatory telescope. In
addition to the previously known expansion of the system of bright filaments
with a velocity of 100-200km/s, we detected weak high-velocity features in the
H-alpha line at least up to velocities of 400-450km/s. We analyzed the
morphology of the PWN in the H-alpha, [SII], and [OIII] lines using HST data
and discuss its nature. The shape of the central filamentary shell, which is
determined by the emission in the [OIII] line and in the radio continuum, is
shown to be consistent with the bow-shock model for a significant (about 60
degrees) inclination of the pulsar's velocity vector to the plane of the sky.
In this case, the space velocity of the pulsar is twice higher than its
tangential velocity, i.e., it reaches ~500 km/s, and PSR B1951+32 is the first
pulsar whose line-of-sight velocity (of about 400 km/s) has been estimated from
the PWN observations. The shell-like H-alpha-structures outside the bow shock
front in the east and the west may be associated with both the pulsar's jets
and the pulsar-wind breakthrough due to the layered structure of the extended
CTB 80 shell.Comment: to appear in Astronomy Letters, 12 pages, 6 postscript figures, two
in colour; for a version with high resolution figures see
http://www.sao.ru/hq/grb/team/vkom/CTB80_fine.pd
New Wolf-Rayet star and its circumstellar nebula in Aquila
We report the discovery of a new Wolf-Rayet star in Aquila via detection of
its circumstellar nebula (reminiscent of ring nebulae associated with late WN
stars) using the Spitzer Space Telescope archival data. Our spectroscopic
follow-up of the central point source associated with the nebula showed that it
is a WN7h star (we named it WR121b). We analyzed the spectrum of WR121b by
using the Potsdam Wolf-Rayet (PoWR) model atmospheres, obtaining a stellar
temperature of ~ 50 kK. The stellar wind composition is dominated by helium
with ~ 20 per cent of hydrogen. The stellar spectrum is highly reddened
(E_{B-V} = 2.85 mag). Adopting an absolute magnitude of M_v = -5.7, the star
has a luminosity of log L/Lsun = 5.75 and a mass-loss rate of 10^{-4.7}
Msun/yr, and resides in a distance of 6.3 kpc. We searched for a possible
parent cluster of WR121b and found that this star is located at ~ 1 degree from
the young star cluster embedded in the giant HII region W43 (containing a
WN7+a/OB? star -- WR121a). We also discovered a bow shock around the O9.5III
star ALS9956, located at ~ 0.5 degree from the cluster. We discuss the
possibility that WR121b and ALS9956 are runaway stars ejected from the cluster
in W43.Comment: 9 pages, 7 figures, accepted to MNRA
The Southern Galactic Plane Survey: The Test Region
The Southern Galactic Plane Survey (SGPS) is a project to image the HI line
emission and 1.4 GHz continuum in the fourth quadrant of the Milky Way at high
resolution using the Australia Telescope Compact Array (ATCA) and the Parkes
Radio Telescope. In this paper we describe the survey details and goals,
present lambda 21-cm continuum data, and discuss HI absorption and emission
characteristics of the SGPS Test Region (325.5 deg < l < 333.5 deg; -0.5 deg <
b < +3.5 deg). We explore the effects of massive stars on the interstellar
medium (ISM) through a study of HI shells and the HI environments of HII
regions and supernova remnants. We find an HI shell surrounding the HII region
RCW 94 which indicates that the region is embedded in a molecular cloud. We
give lower limits for the kinematic distances to SNRs G327.4+0.4 and G330.2+1.0
of 4.3 kpc and 4.9 kpc, respectively. We find evidence of interaction with the
surrounding HI for both of these remnants. We also present images of a possible
new SNR G328.6-0.0. Additionally, we have discovered two small HI shells with
no counterparts in continuum emission.Comment: 17 pages, 7 embedded EPS figures, 10 low-res jpeg figures, uses
emulateapj5.sty. Accepted for publication in the Astrophysical Journal.
Version with all full resolution figures embedded is available at
http://www.astro.umn.edu/~naomi/sgps/papers/SGPS.ps.g
Pre-Main Sequence stars in the star forming complex Sh 2-284
Located at the galactic anticenter, Sh 2-284 is a HII region which harbors
several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young
cluster, is one of these. Given its association with Sh 2-284, it is reasonable
to assume the low metallicity for the whole HII region. Sh~2-284 is expected to
host a significant population of Pre-Main Sequence (PMS) stars of both low and
intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars
by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and
complemented with additional optical and infrared observations. In this survey
we selected and characterized 23 PMS objects. We derived the effective
temperature, the spectral energy distribution and luminosity of these objects;
using theoretical PMS evolutionary tracks, with the appropriate metallicity, we
estimated the mass and the age of the studied objects. We also estimated a
distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars.
From the age determination we concluded that triggered star formation is in act
in this region. Our results show that a significant fraction of the young
stellar objects (YSOs) may have preserved their disk/envelopes, in contrast
with what is found in other recent studies of low-metallicity star forming
regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified
8 stars which are good candidates to pulsators of the delta Scuti type.Comment: accepted for publication in MNRA
Revealing evolved massive stars with Spitzer
Massive evolved stars loss a large fraction of their mass via copious stellar
wind or instant outbursts and during certain evolutionary phases they can be
identified via the presence of their circumstellar nebulae. In this paper, we
present the results of search for compact nebulae (reminiscent of circumstellar
nebulae around evolved massive stars) using archival 24 m data obtained
with the Multiband Imaging Photometer for Spitzer. We discovered 115 nebulae,
most of which bear a striking resemblance to the circumstellar nebulae
associated with Luminous Blue Variables (LBVs) and late WN-type (WNL)
Wolf-Rayet (WR) stars in the Milky Way and the Large Magellanic Cloud (LMC). We
interpret this similarity as an indication that the central stars of detected
nebulae are either LBVs or related evolved massive stars. Our interpretation is
supported by follow-up spectroscopy of two dozens of these central stars, most
of which turns out to be either candidate LBVs (cLBVs), blue supergiants or WNL
stars. We expect that the forthcoming spectroscopy of the remaining objects
from our list, accompanied by the spectrophotometric monitoring of the already
discovered cLBVs, will further increase the known population of Galactic LBVs,
which in turn would have profound consequences for better understanding the LBV
phenomenon and its role in the transition between hydrogen burning O stars and
helium burning WR stars. We also report the detection of an arc-like structure
attached to the cLBV HD326823 and an arc associated with the LBV R99 (HD269445)
in the LMC.Comment: 9 pages, 10 figures, accepted to MNRA