380 research outputs found
The Instability Strip for Pre--Main-Sequence Stars
We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars
by means of linear and nonlinear calculations. The equilibrium models were
taken from models evolved from the protostellar birthline to the ZAMS for
masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to
define the instability strip of PMS stars in the HR diagram. These models are
used to constrain the internal structure of young stars and to test
evolutionary models. We compare our results with observations of the best case
of a pulsating young star, HR~5999, and we also identify possible candidates
for pulsational variability among known Herbig Ae/Be stars which are located
within or close to the instability strip boundaries.Comment: 14 pages, three postscript figures, accepted for publication on the
Astrophysical Journal Letter
The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73
We report the discovery of pulsed X-ray emission from the compact source 1E
1841-045, using data obtained with the Advanced Satellite for Cosmology and
Astrophysics. The X-ray source is located in the center of the small-diameter
supernova remnant (SNR) Kes 73 and is very likely to be the compact
stellar-remnant of the supernova which formed Kes 73. The X-rays are pulsed
with a period of ~ 11.8 s, and a sinusoidal modulation of roughly 30 %. We
interpret this modulation to be the rotation period of an embedded neutron
star, and as such would be the longest spin period for an isolated neutron star
to-date. This is especially remarkable since the surrounding SNR is very young,
at ~ 2000 yr old. We suggest that the observed characteristics of this object
are best understood within the framework of a neutron star with an enormous
dipolar magnetic field, B ~ 8x10^14 G
Updated Information on the Local Group
The present note updates the information published in my recent monograph on
\underline{The Galaxies of the Local Group}. Highlights include (1) the
addition of the newly discovered Cetus dwarf spheroidal as a certain member of
the Local Group, (2) an improved distance for SagDIG, which now places this
object very close to the edge of the Local Group zero-velocity surface, (3)
more information on the evolutionary histories of some individual Local Group
members, and (4) improved distance determinations to, and luminosities for, a
number of Local Group members. These data increase the number of certain (or
probable) Local Group members to 36. The spatial distribution of these galaxies
supports Hubble's claim that the Local Group ``is isolated in the general
field.'' Presently available evidence suggests that star formation continued
much longer in many dwarf spheroidals than it did in the main body of the
Galactic halo. It is suggested that ``young'' globular clusters, such as
Ruprecht 106, might have formed in now defunct dwarf spheroidals. Assuming
SagDIG, which is the most remote Local Group galaxy, to lie on, or just inside,
the zero-velocity surface of the Local Group yields a dynamical age \gtrsim
17.9 \pm 2.7 Gyr.Comment: 19 pages, 1 figure, to be published in the April 2000 issue of PAS
Multiperiodicity in the newly discovered mid-late Be star V2104 Cygni
We obtained the first long, homogenous time-series of V2104Cyg, consisting of
679 datapoints, with the uvbybeta photometers of Sierra Nevada and San Pedro
Martir Observatories with the aim to detect and subsequently interpret the
intrinsic frequencies of this previously unstudied variable star, which turned
out to be a Be star. We try to figure out its place among the variable B stars
on the upper Main Sequence. In order to obtain additional information on
physical parameters we collected a few spectra with the ELODIE and FIES
instruments. We searched for frequencies in the uvby passbands using 2
different frequency analysis methods and used the S/N>4 criterion to select the
significant periodicities. We obtained an estimate of the physical parameters
of the underlying B star of spectral type between B5 and B7, by correcting for
the presence of a circumstellar disk, using a formalism based on the strenght
of the Halpha line emission. We detected 3 independent frequencies with
amplitudes below 0.01mag, f1 = 4.7126 c/d, f2 = 2.2342 c/d and f3 = 4.671 c/d,
and discovered that V2104Cyg is a Be star. The fast rotation (vsini=290+/-10
km/s, and 27<i<45) hampered the investigation of the associated pulsational
parameters l. Nevertheless, the most plausible explanation for the observed
variability of this mid-late type Be star is a non-radial pulsation model. This
paper is based on observations obtained at the Observatorio Astronomico
Nacional San Pedro Martir (Mexico), Observatorio de Sierra Nevada (Spain),
Observatoire de Haute Provence (France), and on observations made with the
Nordic Optical Telescope, Observatorio Roque de los Muchachos, La Palma, Spain.Comment: 7 pages, 4 figures, A&A accepte
Keck and ESO-VLT View of the Symmetry of the Ejecta of the XRF/SN 2006aj
Nebular-phase spectra of SN 2006aj, which was discovered in coincidence with
X-ray flash 060218, were obtained with Keck in 2006 July and the Very Large
Telescope in 2006 September. At the latter epoch spectropolarimetry was also
attempted, yielding an upper limit of ~ 2% for the polarization. The spectra
show strong emission lines of [OI] and MgI], as expected from a Type Ic
supernova, but weak CaII lines. The [FeII] lines that were strong in the
spectra of SN 1998bw are much weaker in SN 2006aj, consistent with the lower
luminosity of this SN. The outer velocity of the line-emitting ejecta is ~ 8000
km/s in July and ~ 7400 km/s in September, consistent with the relatively low
kinetic energy of expansion of SN 2006aj. All emission lines have similar
width, and the profiles are symmetric, indicating that no major asymmetries are
present in the ejecta at the velocities sampled by the nebular lines (v < 8000
km/s), except perhaps in the innermost part. The spectra were modelled with a
non-LTE code. The mass of 56Ni required to power the emission spectrum is ~
0.20 Msun, in excellent agreement with the results of early light curve
modelling. The oxygen mass is ~ 1.5 Msun, again much less than in SN 1998bw but
larger by ~ 0.7 Msun than the value derived from the early-time modelling. The
total ejected mass is ~ 2 Msun below 8000 km/s. This confirms that SN 2006aj
was only slightly more massive and energetic than the prototypical Type Ic SN
1994I, but also indicates the presence of a dense inner core, containing ~ 1
Msun of mostly oxygen and carbon. The presence of such a core is inferred for
all broad-lined SNe Ic. This core may have the form of an equatorial
oxygen-dominated region, but it is too deep to affect the early light curve and
too small to affect the late polarization spectrum.Comment: 20 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Spectropolarimetry of the Type IIb Supernova 2001ig
We present spectropolarimetric observations of the Type IIb SN 2001ig in NGC
7424; conducted with the ESO VLT FORS1 on 2001 Dec 16, 2002 Jan 3 and 2002 Aug
16 or 13, 31 and 256 days post-explosion. These observations are at three
different stages of the SN evolution: (1) The hydrogen-rich photospheric phase,
(2) the Type II to Type Ib transitional phase and (3) the nebular phase. At
each of these stages, the observations show remarkably different polarization
properties as a function of wavelength. We show that the degree of interstellar
polarization is 0.17%. The low intrinsic polarization (~0.2%) at the first
epoch is consistent with an almost spherical (<10% deviation from spherical
symmetry) hydrogen dominated ejecta. Similar to SN 1987A and to Type IIP SNe, a
sharp increase in the degree of the polarization (~1%) is observed when the
outer hydrogen layer becomes optically thin by day 31; only at this epoch is
the polarization well described by a ``dominant axis.'' The polarization angle
of the data shows a rotation through ~40 degrees between the first and second
epochs, indicating that the asymmetries of the first epoch were not directly
coupled with those observed at the second epoch. For the most polarized lines,
we observe wavelength-dependent loop structures in addition to the dominant
axis on the Q-U plane. We show that the polarization properties of Type IIb SNe
are roughly similar to one another, but with significant differences arising
due to line blending effects especially with the high velocities observed for
SN 2001ig. This suggests that the geometry of SN 2001ig is related to SN 1993J
and that these events may have arisen from a similar binary progenitor system.Comment: 42 pages, 12 figures (figs. 11 and 12 are both composed of four
subpanels, figs. 6,7,8,11 and 12 are in color, fig. 1 is low res and a high
res version is available at http://www.as.utexas.edu/~jrm/), ApJ Accepte
The Axially Symmetric Ejecta of Supernova 1987A
Extensive early observations proved that the ejecta of supernova 1987A (SN
1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble
Space Telescope has resolved the rapidly expanding ejecta. The late-time images
and spectroscopy provide a geometrical picture that is consistent with early
observations and suggests a highly structured, axially symmetric geometry. We
present here a new synthesis of the old and new data. We show that the Bochum
event, presumably a clump of Ni, and the late-time image, the locus of
excitation by Ti, are most naturally accounted for by sharing a common
position angle of about 14\degree, the same as the mystery spot and early
speckle data on the ejecta, and that they are both oriented along the axis of
the inner circumstellar ring at 45\degree to the plane of the sky. We also
demonstrate that the polarization represents a prolate geometry with the same
position angle and axis as the early speckle data and the late-time image and
hence that the geometry has been fixed in time and throughout the ejecta. The
Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in
spatially resolved HST spectra of the ejecta can be consistently integrated
into this geometry. The radioactive clump is deduced to fall approximately
along the axis of the inner circumstellar ring and therefore to be redshifted
in the North whereas the [Ca II]/[O II] 7300 \AA emission is redshifted in the
South. We present a jet-induced model for the explosion and argue that such a
model can account for many of the observed asymmetries. In the jet models, the
oxygen and calcium are not expected to be distributed along the jet, but
primarily in an expanding torus that shares the plane and northern blue shift
of the inner circumstellar ring.Comment: To Appear in Ap
Improvement of the CORS method for Cepheids radii determination based on Stromgren photometry
In this paper we present a modified version of the CORS method based on a new
calibration of the Surface Brightness function in the Stromgren photometric
system. The method has been tested by means of synthetic light and radial
velocity curves derived from nonlinear pulsation models. Detailed simulations
have been performed to take into account the quality of real observed curves as
well as possible shifts between photometric and radial velocity data. The
method has been then applied to a sample of Galactic Cepheids with Stromgren
photometry and radial velocity data to derive the radii and a new PR relation.
As a result we find log R = (1.19 +- 0.09) + (0.74 +- 0.11) log P (r.m.s=0.07).
The comparison between our result and previous estimates in the literature is
satisfactory. Better results are expected from the adoption of improved model
atmosphere grids.Comment: 13 pages including 12 postscript figures and 3 jpeg figures; accepted
for publication on A&
Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components
We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen.
The central regions of the cluster were covered with a mosaic of F435W, F625W,
and F658N-band data collected with ACS/HST. The outer reaches were covered with
a large set of U,B,V,I-band data collected with the [email protected] ESO/MPI telescope.
The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB
stars and ~12,500 stars brighter than the subgiant branch and fainter than the
RGB bumps. The HB morphology changes with the radial distance. The relative
number of extreme HB stars decreases from ~30% to ~21% when moving from the
center toward the outer regions of the cluster, while the fraction of less hot
HB stars increases from ~62% to ~72%. We performed a detailed comparison
between observed ratios of different stellar tracers and predictions based on
canonical evolutionary models with a primordial helium (Y=0.23) content and
metal abundances (Z=0.0002,0.001) that bracket the observed spread in
metallicity of Omega Cen stars. We found that the empirical star counts of HB
stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB
stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB
compared with the rate of RG and MSTO stars supports the evidence that we are
facing a true excess of HB stars. The same comparison was performed by assuming
a mix of stellar populations made with 70% of canonical stars and 30% of
He-enhanced stars. The discrepancy between theory and observations decreases by
a factor of two when compared with rates predicted by canonical He content
models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed.
Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and
30% (Y=0.33) larger than predicted lifetime ratios.Comment: 54 pages, 17 figures,to be published in ApJ, see link at
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