2,060 research outputs found
A new HCN maser in IRAS 15082-4808
We have identified a new vibrational HCN maser at 89.087 GHz in the
asymptotic giant branch (AGB) star IRAS 15082-4808, a maser which is thought to
trace the innermost region of an AGB envelope. The observations of this maser
at three epochs are presented: two positive detections and one null detection.
The line profile has varied between the positive detections, as has the
intensity of the maser. The major component of the maser is found to be offset
by -2.0+/-0.9 km/s with respect to the systemic velocity of the envelope, as
derived from the 88.631 GHz transition of HCN. Similar blueshifts are measured
in the other 9 sources where this maser has been detected. Maser variability
with pulsation phase has been investigated for the first time using the 10
stars now available. Comparisons with AGB model atmospheres constrain the
position of the formation region of the maser to the region between the
pulsation shocks and the onset of dust acceleration, between 2 and 4 stellar
radii.Comment: 11 pages, 10 figures, accepted MNRAS, minor correction to equation
The Stellar Populations of NGC 3109: Another Dwarf Irregular Galaxy with a Population II Stellar Halo
We have obtained V and I-band photometry for about 17500 stars in the field
of the dwarf irregular galaxy NGC3109, located in the outskirts of the Local
Group. The photometry allows us to study the stellar populations present inside
and outside the disk of this galaxy. From the VI color-magnitude diagram we
infer metallicities and ages for the stellar populations in the main body and
in the halo of NGC3109. The stars in the disk of this galaxy have a wide
variety of ages, including very young stars with approximately 10^7 yr. Our
main result is to establish the presence of a halo consisting of population II
stars, extending out to about 4.5 arcmin (or 1.8 kpc) above and below the plane
of this galaxy. For these old stars we derive an age of > 10 Gyr and a
metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a deep luminosity function,
obtaining an accurate distance modulus (m-M)_0 = 25.62 +/- 0.1 for this galaxy
based on the I-magnitude of the red giant branch (RGB) tip and adopting E(V-I)
= 0.05.Comment: Accepted for publication in the Astronomical Journal 23 pages, latex,
12 Figures (Fig 1 not available in electronic format
Mode switching in the nearby Mira-like variable R Doradus
We discuss visual observations spanning nearly 70 years of the nearby
semiregular variable R Doradus. Using wavelet analysis, we show that the star
switches back and forth between two pulsation modes having periods of 332 days
and about 175 days, the latter with much smaller amplitude. Comparison with
model calculations suggests that the two modes are the first and third radial
overtone, with the physical diameter of the star making fundamental mode
pulsation unlikely. The mode changes occur on a timescale of about 1000 d,
which is too rapid be related to a change in the overall thermal structure of
the star and may instead be related to weak chaos.
The Hipparcos distance to R Dor is 62.4 +/- 2.8 pc which, taken with its
dominant 332-day period, places it exactly on the period-luminosity relation of
Miras in the Large Magellanic Cloud. Our results imply first overtone pulsation
for all Miras which fall on the P-L relation. We argue that semiregular
variables with long periods may largely be a subset of Miras and should be
included in studies of Mira behaviour. The semiregulars may contain the
immediate evolutionary Mira progenitors, or stars may alternate between periods
of semiregular and Mira behaviour.Comment: 12 pages, latex with figures, accepted by MNRA
Third-dredge-up oxygen in planetary nebulae
The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf
galaxy appear to result from nearly twin stars, except that third-dredge-up
carbon is more abundant in He 2-436. A thorough photoionization-model analysis
implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436,
indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with
respect to carbon is (7 +/- 4)%. Excess nitrogen in Wray 16-423 suggests third
dredge-up of late CN-cycle products even in these low-mass,
intermediate-metallicity stars.Comment: To appear in Astron. Astrophys. Lett. (Latex, 5 pages, 1 postscript
figure
Millimeter polarisation of the protoplanetary nebula OH 231.8+4.2: A follow-up study with CARMA
In order to investigate the characteristics and influence of the magnetic
field in evolved stars, we performed a follow-up investigation of our previous
submillimeter analysis of the proto-planetary nebula (PPN) OH 231.8+4.2 (Sabin
et al. 2014), this time at 1.3mm with the CARMA facility in polarisation mode
for the purpose of a multi-scale analysis. OH 231.8+4.2 was observed at ~2.5"
resolution and we detected polarised emission above the 3-sigma threshold (with
a mean polarisation fraction of 3.5 %). The polarisation map indicates an
overall organised magnetic field within the nebula. The main finding in this
paper is the presence of a structure mostly compatible with an ordered toroidal
component that is aligned with the PPN's dark lane. We also present some
alternative magnetic field configuration to explain the structure observed.
These data complete our previous SMA submillimeter data for a better
investigation and understanding of the magnetic field structure in OH
231.8+4.2.Comment: 7 pages, 5 figures, 2 tables. Accepted for publication in MNRA
Modelling the alumina abundance of oxygen-rich evolved stars in the Large Magellanic Cloud
In order to determine the composition of the dust in the circumstellar
envelopes of oxygen-rich asymptotic giant branch (AGB) stars we have computed a
grid of modust radiative-transfer models for a range of dust compositions,
mass-loss rates, dust shell inner radii and stellar parameters. We compare the
resulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec
Large Magellanic Cloud (LMC) sample, finding good overall agreement for stars
with a mid-infrared excess. We use these models to fit a sample of 37 O-rich
AGB stars in the LMC with optically thin circumstellar envelopes, for which
535-m Spitzer infrared spectrograph (IRS) spectra and broadband
photometry from the optical to the mid-infrared are available. From the
modelling, we find mass-loss rates in the range to
M, and we show that a grain
mixture consisting primarily of amorphous silicates, with contributions from
amorphous alumina and metallic iron provides a good fit to the observed
spectra. Furthermore, we show from dust models that the AKARI [11][15]
versus [3.2][7] colour-colour diagram, is able to determine the fractional
abundance of alumina in O-rich AGB stars.Comment: 22 pages, 17 figures, accepted MNRA
Detection of the Central Star of the Planetary Nebula NGC 6302
NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be >200,000K to photoionize the nebula, and has been
suggested to be up to ~ 400,000K. On account of the dense dust and molecular
disc, the central star has not convincingly been directly imaged until now. NGC
6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as
part of the Servicing Mission 4 Early Release Observations. The central star is
directly detected for the first time, and is situated at the nebula centre on
the foreground side of the tilted equatorial disc. The magnitudes of the
central star have been reliably measured in two filters(F469N and F673N).
Assuming a hot black body, the reddening has been measured from the
(4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main
sequence binary companion can be excluded. The position of the star on the HR
diagram suggests a fairly massive PN central star of about 0.64,M_sun close to
the white dwarf cooling track. A fit to the evolutionary tracks for
(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;
however the luminosity and temperature remain uncertain. The model tracks
predict that the star is rapidly evolving, and fading at a rate of almost 1 %
per year. Future observations could test this prediction.Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted
on 19.10.200
Oscillations of a gas pocket on a liquid-covered solid surface
The dynamic response of a gas bubble entrapped in a cavity on the surface of
a submerged solid subject to an acoustic field is investigated in the linear
approximation. We derive semi-analytical expressions for the resonance
frequency, damping and interface shape of the bubble. For the liquid phase, we
consider two limit cases: potential flow and unsteady Stokes flow. The
oscillation frequency and interface shape are found to depend on two
dimensionless parameters: the ratio of the gas stiffness to the surface tension
stiffness, and the Ohnesorge number, representing the relative importance of
viscous forces. We perform a parametric study and show, among others, that an
increase in the gas pressure or a decrease in the surface tension leads to an
increase in the resonance frequency until an asymptotic value is reached
Subarcsecond Submillimeter Imaging of the Ultracompact HII Region G5.89-0.39
We present the first subarcsecond submillimeter images of the enigmatic
ultracompact HII region (UCHII) G5.89-0.39. Observed with the SMA, the 875
micron continuum emission exhibits a shell-like morphology similar to longer
wavelengths. By using images with comparable angular resolution at five
frequencies obtained from the VLA archive and CARMA, we have removed the
free-free component from the 875 micron image. We find five sources of dust
emission: two compact warm objects (SMA1 and SMA2) along the periphery of the
shell, and three additional regions further out. There is no dust emission
inside the shell, supporting the picture of a dust-free cavity surrounded by
high density gas. At subarcsecond resolution, most of the molecular gas tracers
encircle the UCHII region and appear to constrain its expansion. We also find
G5.89-0.39 to be almost completely lacking in organic molecular line emission.
The dust cores SMA1 and SMA2 exhibit compact spatial peaks in optically-thin
gas tracers (e.g. 34SO2), while SMA1 also coincides with 11.9 micron emission.
In CO(3-2), we find a high-velocity north/south bipolar outflow centered on
SMA1, aligned with infrared H2 knots, and responsible for much of the maser
activity. We conclude that SMA1 is an embedded intermediate mass protostar with
an estimated luminosity of 3000 Lsun and a circumstellar mass of ~1 Msun.
Finally, we have discovered an NH3 (3,3) maser 12 arcsec northwest of the UCHII
region, coincident with a 44 GHz CH3OH maser, and possibly associated with the
Br gamma outflow source identified by Puga et al. (2006).Comment: 41 pages, 11 figures, published in The Astrophysical Journal (2008)
Volume 680, Issue 2, pp. 1271-1288. An error in the registration of the
marker positions in Figure 11 has been corrected in this versio
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