1,055 research outputs found
The Unique Frequency Spectrum of the Blazhko RRc Star LS Her
The Blazhko effect in RR Lyrae stars is still poorly understood
theoretically. Stars with multiple Blazhko periods or in which the Blazhko
effect itself varies are particularly challenging. This study investigates the
Blazhko effect in the RRc star LS Her. Detailed VRI CCD photometry has been
performed on 63 nights during six months. LS Her is confirmed to have a Blazhko
period of 12.75+/-0.02 days. However, where normally the side frequencies of
the Blazhko triplet are expected, an equidistant group of three frequencies is
found on both sides of the main pulsation frequency. As a consequence the
period and amplitude of the Blazhko effect itself vary in a cycle of 109+/-4
days. LS Her is a unique object turning out to be very important in the
verification of the theories for the Blazhko effect.Comment: Accepted for publication in MNRA
Discovery of a Transition to Global Spin-up in EXO 2030+375
EXO 2030+375, a 42-second transient X-ray pulsar with a Be star companion,
has been observed to undergo an outburst at nearly every periastron passage for
the last 13.5 years. From 1994 through 2002, the global trend in the pulsar
spin frequency was spin-down. Using RXTE data from 2003 September, we have
observed a transition to global spin-up in EXO 2030+375. Although the spin
frequency observations are sparse, the relative spin-up between 2002 June and
2003 September observations, along with an overall brightening of the outbursts
since mid 2002 observed with the RXTE ASM, accompanied by an increase in
density of the Be disk, indicated by infrared magnitudes, suggest that the
pattern observed with BATSE of a roughly constant spin frequency, followed by
spin-up, followed by spin-down is repeating. If so this pattern has
approximately an 11 year period, similar to the 15 +/- 3 year period derived by
Wilson et al. (2002) for the precession period of a one-armed oscillation in
the Be disk. If this pattern is indeed repeating, we predict a transition from
spin-up to spin-down in 2005.Comment: Accepted for publication in ApJ Letters, 4 pages, 5 figures, using
emulateapj.cl
The Nature of the Compact Supernova Remnants in Starburst Galaxies
Radio observations of starburst regions in galaxies have revealed groups of
compact nonthermal sources that may be radiative supernova remnants expanding
in the interclump medium of molecular clouds. Because of the high pressure in
starburst regions, the interclump medium may have a density ~ 10^3 H atoms
cm^{-3} in a starburst nucleus like M82 and ~ 10^4 H atoms cm^{-3} in an
ultraluminous galaxy like Arp 220. In M82, our model can account for the sizes,
the slow evolution, the high radio luminosities, and the low X-ray luminosities
of the sources. We predict expansion velocities ~ 500 km/s, which is slower
than the one case measured by VLBI techniques. Although we predict the remnants
to be radiative, the expected radiation is difficult to detect because it is at
infrared wavelengths and the starburst is itself very luminous; one detection
possibility is broad [OI] 63 micron line emission at the positions of the radio
remnants. The more luminous and compact remnants in Arp 220 can be accounted
for by the higher molecular cloud density. In our model, the observed remnants
lose most of the supernova energy to radiation. Other explosions in a lower
density medium may directly heat a hot, low density interstellar component,
leading to the superwinds that are associated with starburst regions.Comment: 11 pages, 1 figure, ApJ submitte
The Nature of Starburst Activity in M82
We present new evolutionary synthesis models of M82 based mainly on
observations consisting of near-infrared integral field spectroscopy and
mid-infrared spectroscopy. The models incorporate stellar evolution, spectral
synthesis, and photoionization modeling, and are optimized for 1-45 micron
observations of starburst galaxies. The data allow us to model the starburst
regions on scales as small as 25 pc. We investigate the initial mass function
(IMF) of the stars and constrain quantitatively the spatial and temporal
evolution of starburst activity in M82. We find a typical decay timescale for
individual burst sites of a few million years. The data are consistent with the
formation of very massive stars (> 50-100 Msun) and require a flattening of the
starburst IMF below a few solar masses assuming a Salpeter slope at higher
masses. Our results are well matched by a scenario in which the global
starburst activity in M82 occurred in two successive episodes each lasting a
few million years, peaking about 10 and 5 Myr ago. The first episode took place
throughout the central regions of M82 and was particularly intense at the
nucleus while the second episode occurred predominantly in a circumnuclear ring
and along the stellar bar. We interpret this sequence as resulting from the
gravitational interaction M82 and its neighbour M81, and subsequent bar-driven
evolution. The short burst duration on all spatial scales indicates strong
negative feedback effects of starburst activity, both locally and globally.
Simple energetics considerations suggest the collective mechanical energy
released by massive stars was able to rapidly inhibit star formation after the
onset of each episode.Comment: 48 pages, incl. 16 Postscript figures; accepted for publication in
the Astrophysical Journa
ISO LWS Spectroscopy of M82: A Unified Evolutionary Model
We present the first complete far-infrared spectrum (43 to 197 um) of M82,
the brightest infrared galaxy in the sky, taken with the Long Wavelength
Spectrometer of the Infrared Space Observatory (ISO). We detected seven fine
structure emission lines, [OI] 63 and 145 um, [OIII] 52 and 88 um, [NII] 122
um, [NIII] 57 um and [CII] 158 um, and fit their ratios to a combination
starburst and photo-dissociation region (PDR) model. The best fit is obtained
with HII regions with n = 250 cm^{-3} and an ionization parameter of 10^{-3.5}
and PDRs with n = 10^{3.3} cm^{-3} and a far-ultraviolet flux of G_o =
10^{2.8}. We applied both continuous and instantaneous starburst models, with
our best fit being a 3-5 Myr old instantaneous burst model with a 100 M_o
cut-off. We also detected the ground state rotational line of OH in absorption
at 119.4 um. No excited level OH transitions are apparent, indicating that the
OH is almost entirely in its ground state with a column density ~ 4x10^{14}
cm^{-2}. The spectral energy distribution over the LWS wavelength range is well
fit with a 48 K dust temperature and an optical depth, tau_{Dust} proportional
to lambda^{-1}.Comment: 23 pages, 4 figures, accepted by ApJ, Feb. 1, 199
Does the Milky Way Produce a Nuclear Galactic Wind?
We detect high-velocity absorbing gas using Hubble Space Telescope and Far
Ultraviolet Spectroscopic Explorer medium resolution spectroscopy along two
high-latitude AGN sight lines (Mrk 1383 and PKS 2005-489) above and below the
Galactic Center (GC). These absorptions are most straightforwardly interpreted
as a wind emanating from the GC which does not escape from the Galaxy's
gravitational potential. Spectra of four comparison B stars are used to
identify and remove foreground velocity components from the absorption-line
profiles of O VI, N V, C II, C III, C IV, Si II, Si III, and Si IV. Two
high-velocity (HV) absorption components are detected along each AGN sight
line, three redshifted and one blueshifted. Assuming that the four HV features
trace a large-scale Galactic wind emanating from the GC, the blueshifted
absorber is falling toward the GC at a velocity of 250 +/- 20 km/s, which can
be explained by "Galactic fountain" material that originated in a bound
Galactic wind. The other three absorbers represent outflowing material; the
largest derived outflow velocity is +250 +/- 20 km/s, which is only 45% of the
velocity necessary for the absorber to escape from its current position in the
Galactic gravitational potential. All four HV absorbers are found to reach the
same maximum height above the Galactic plane (|z_max| = 12 +/- 1 kpc), implying
that they were all ejected from the GC with the same initial velocity. The
derived metallicity limits of >10-20% Solar are lower than expected for
material recently ejected from the GC unless these absorbers also contain
significant amounts of hotter gas in unseen ionization stages.Comment: 39 pages, 3 figures, ApJ accepte
- âŠ