109 research outputs found
The large- and small-scale Ca IIK structure of the Milky Way from observations of Galactic and Magellanic sightlines
Aims: By utilising spectra of early-type stellar probes of known distances in
the same region of the sky, the large and small-scale (pc) structure of the
Galactic ISM can be investigated. This paper determines the variation in line
strength of CaII at 3933.661 A, as a function of probe separation for a sample
of stars, including many sightlines in the Magellanic Clouds. Methods:
FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic &
4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column
density and line width of IS Galactic Ca for a total of 657 stars, of which 443
are Magellanic sightlines. In each cluster there are 43-110 stars observed.
Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic
early-type stars are presented and combined with data from the literature to
study the Ca column density/parallax relationship. Results: For the four
Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K
EW over transverse scales from 0.05-9 pc is found to vary by factors of
1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in the
optically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, the
min and max reduced EW for MW gas is found to lie in the range 35-125 mA &
30-160 mA for CaII K and NaI D, respectively. The range is consistent with a
simple model of the ISM published by van Loon et al. (2009) consisting of
spherical cloudlets of filling factor 0.3, although other geometries are not
ruled out. Finally, the derived functional form for parallax and CaII column
density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived
parallax is 25 per cent lower than predicted by Megier et al. (2009) at a
distance of 100 pc and 15% lower at a distance of 200 pc, reflecting
inhomogeneity in the CaII distribution in the different sightlines studied.Comment: The full version including online material is available via the
Astronomy and Astrophysics website
http://www.aanda.org/articles/aa/olm/2015/10/aa25190-14/aa25190-14.htm
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
HI Properties of Low Luminosity Star-Forming Galaxies in the KPNO International Spectroscopic Survey
New HI observations are presented for a complete sample of 109 low luminosity
star-forming galaxies taken from the KPNO International Spectroscopic Survey
(KISS), the first CCD-based wide-field objective-prism survey for emission-line
galaxies. This sample consists of all star-forming galaxies with M_B > -18.0
and cz < 11,000 km/s from the first Halpha-selected survey list. Overall, 97
out of 109 galaxies have been detected in HI. We confirm the weak trend of
increasing gas richness with decreasing luminosity found by previous authors.
Gas richness is also shown to be weakly anti-correlated with metallicity. The
dependence of star formation rates (SFRs) and HI gas depletion timescales on
metallicity is examined. The median solar metallicity based SFR and gas
depletion timescale are 0.1639 M_sun/yr and 5 Gyrs, respectively. Corrections
for variations in metallicity decreases SFRs by about 0.5 dex and increases gas
depletion timescales by an average of about 8 Gyrs. The majority of galaxies in
this sample still have large reservoirs of HI gas, and despite their large
current star formation rates, could have formed stars in a quasi-continuous
manner for a Hubble time. Finally, we present the first HI mass function for
low luminosity star-forming galaxies and show that this subpopulation
contributes 10-15% of the overall HI density in the local universe. We conclude
that if the HI mass function of the Universe does indeed have a steeply rising
low-mass slope as suggested by previous authors, it is not due to the
population of low luminosity star-forming galaxies.Comment: 27 pages, 13 figures, 4 tables. Accepted for publication in the
Astronomical Journa
Large Interstellar Polarisation Survey. LIPS I: FORS2 spectropolarimetry in the Southern Hemisphere
Polarimetric studies of light transmitted through interstellar clouds may
give constraints on the properties of the interstellar dust grains.
Traditionally, broadband linear polarisation (BBLP) measurements have been
considered an important diagnostic tool for the study of the interstellar dust,
while comparatively less attention has been paid to spectropolarimetric
measurements. However, spectropolarimetry may offer stronger constraints than
BBLP, for example by revealing narrowband features, and by allowing us to
distinguish the contribution of dust from the contribution of interstellar gas.
Therefore, we have decided to carry out a Large Interstellar Polarisation
Survey (LIPS) using spectropolarimetric facilities in both hemispheres. Here we
present the results obtained in the Southern Hemisphere with the FORS2
instrument of the ESO Very Large Telescope. Our spectra cover the wavelength
range 380--950\,nm at a spectral resolving power of about 880. We have produced
a publicly available catalogue of 127 linear polarisation spectra of 101
targets. We also provide the Serkowski-curve parameters, as well as the
wavelength gradient of the polarisation position angle for the interstellar
polarisation along 76 different lines of sight. In agreement with previous
literature, we found that the best-fit parameters of the Serkowski-curve are
not independent of each other. However, the relationships that we obtained are
not always consistent with what was found in previous studies.Comment: Accepted by A&A (replaced on 12 October 2017 simply to correct a
Metadata error
CaII K observations of QSOs in the line-of-sight to the Magellanic Bridge
We describe medium-resolution spectroscopic observations taken with the ESO
Multi-Mode Instrument in the CaII K line (3933.661 Angstroms) towards 7 QSOs
located in the line-of-sight to the Magellanic Bridge. At a spectral resolution
R = 6,000, five of the sightlines have a signal-to-noise ratio of 20 or higher.
Definite Ca absorption due to Bridge material is detected towards 3 objects,
with probable detection towards two other sightlines. Gas-phase CaII K Bridge
and Milky Way abundances or lower limits for the all sightlines are estimated
by the use of Parkes 21-cm HI emission line data. These data only have a
spatial resolution of 14 arcminutes compared with the optical observations
which have milli-arcsecond resolution. With this caveat, for the three objects
with sound CaII K detections, we find that the ionic abundance of CaII K
relative to HI, A=log(N(CaK)/N(HI)) for low-velocity Galactic gas ranges from
-8.3 to -8.8 dex, with HI column densities varying from 3-6x10^20 cm^-2. For
Magellanic Bridge gas, the values of A are 0.5 dex higher, ranging from -7.8 to
-8.2 dex, with N(HI)=1-5x10^20 cm^-2. Higher values of A correspond to lower
values of N(HI), although numbers are small. For the sightline towards
B0251--675, the Bridge gas has two different velocities, and in only one of
these is CaII K tentatively detected, perhaps indicating gas of a different
origin or present-day characteristics (such as dust content), although this
conclusion is uncertain and there is the possibility that one of the components
could be related to the Magellanic Stream. Higher signal-to-noise CaII K data
and higher resolution HI data are required to determine whether A changes with
N(HI) over the Bridge and if the implied difference in the metalicity of the
two Bridge components towards B0251-675 is real.Comment: Astronomy and Astrophysics, in pres
Metallicity and Physical Conditions in the Magellanic Bridge
We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h)
based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the
Bridge and a QSO behind it. We derive the column densities of HI (from
Ly\alpha), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the
atomic species, we determine the first gas-phase metallicity of the Magellanic
Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the
other one, a factor 2 or more smaller than the present-day SMC metallicity.
Using the metallicity and N(HI), we show that the Bridge gas along our three
lines of sight is ~70-90% ionized, despite high HI columns, logN(HI)=19.6-20.1.
Possible sources for the ongoing ionization are certainly the hot stars within
the Bridge, hot gas (revealed by OVI absorption), and leaking photons from the
SMC and LMC. From the analysis of CII*, we deduce that the overall density of
the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined
with other recent observational results should motivate new models of the
evolution of the SMC-LMC-Galaxy system.Comment: Accepted for publication in the Ap
FUSE Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundance
We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in
the low density and low metallicity gas of Magellanic Bridge (MB). Toward
DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other
atomic or ionic transitions in absorption, implying the presence of multiple
gas phases in a complex arrangement. The relative abundance pattern in the MB
is attributed to varying degrees of depletion onto dust similar to that of halo
clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha
systems, implying subsequent stellar processing to explain the origin of
nitrogen in the MB. The diffuse molecular cloud in this direction has a low
column density and low molecular fraction. H2 is observed in both the
Magellanic Stream and the MB, yet massive stars form only in the MB, implying
significantly different physical processes between them. In the MB some of the
H2 could have been pulled out from the SMC via tidal interaction, but some also
could have formed in situ in dense clouds where star formation might have taken
place. Toward DGIK975, the presence of neutral, weakly and highly ionized
species suggest that this sight line has also several complex gas phases. The
highly ionized species of O VI, C IV, and Si IV toward both stars have very
broad features, indicating that multiple components of hot gas at different
velocities are present. Several sources (a combination of turbulent mixing
layer, conductive heating, and cooling flows) may be contributing to the
production of the highly ionized gas in the MB. Finally, this study has
confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is
mainly ionized composed of weakly and highly ions. The high ion ratios are
consistent with a radiatively cooling gas in a fountain flow model.Comment: Accepted for publication in the ApJ (October 10, 2002). Added
reference (Gibson et al. 2000
Multiwavelength observations of the M15 intermediate velocity cloud
We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope
multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images,
William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60
and 100 micron coadded images towards the intermediate velocity cloud located
in the general direction of the M15 globular cluster. When combined with
previously-published Arecibo data, the HI gas in the IVC is found to be clumpy,
with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density
(assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness
temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is
warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic
temperature, in the absence of turbulence, of 540 K. There are indications in
the HI data of 2-component velocity structure in the IVC, indicative of
cloudlets. This velocity structure is also tentatively seen in the CaK spectra,
although the SNR is low. The main IVC condensation is detected by WHAM in
H-alpha with intensities uncorrected for Galactic absorption of upto 1.3
Rayleigh, indicating that the cloud is partially ionised. The FWHM of the
ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial
and velocity coincidence of the H-alpha and HI peaks in emission is
qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial
coincidence over a 0.7 degree field, with low and intermediate-velocity gas,
respectively, indicating that the IVC may contain dust.Comment: MNRAS, in pres
CaII K interstellar observations towards early disc and halostars - Paper II; distances to IVCs and HVCs
We compare existing high spectral resolution (R=40,000) CaII K observations
towards 88 mainly B-type stars, and new observations at R=10,000 towards 3
stars, with 21-cm HI emission-line profiles, in order to search for optical
absorption towards known intermediate and high velocity cloud complexes. Given
certain assumptions, limits to the gas phase abundance of CaII are estimated
for the cloud components. We use the data to derive the following distances
from the Galactic plane (z); 1) Tentative lower z-height limits of 2800 pc and
4100 pc towards Complex C using lack of absorption in the spectra of HD 341617
and PG 0855+294. 2) A weak lower z-height of 1400 pc towards Complex WA-WB
using lack of absorption in EC 09470-1433 and weak lower limit of 2470 pc with
EC 09452-1403. 3) An upper z-height of 2470 pc towards a southern intermediate
velocity cloud (IVC) with v_LSR=-55 km/s using PG 2351+198. 4) Detection of a
possible IVC in CaK absorption at v_LSR=+52 km/s using EC 20104-2944. No
associated HI in emission is detected. At this position, normal Galactic
rotation predicts velocities of up to +25 km/s. The detection puts an upper
z-height of 1860 pc to the cloud. 5) Tentative HI and CaK detections towards an
IVC at +70 km/s in the direction of HVC Complex WE, sightline EC 06387-8045,
indicating that the IVC may be at a z-height lower than 1770 pc. 6) Detection
of CaK absorption in the spectrum of PG 0855+294 in the direction of IV20,
indicating that this IVC has a z-height smaller than 4100 pc. 7) A weak lower
z-height of 4300 pc towards a small HVC with v_LSR=+115 km/s at l,b=200,+52,
using lack of absorption in the CaK spectrum of PG 0955+291.Comment: 13 pages, 5 figures. Accepted for publication in MNRAS, May 13 200
The metallicities of UM151, UM408 and A1228+12 revisited
We present the results of new spectrophotometry and heavy element abundance
determinations for 3 dwarf galaxies UM151, UM408 and A1228+12 (RMB132). These
galaxies have been claimed in the literature to have very low metallicities,
corresponding to log(O/H)+12 < 7.65, that are in the metallicity range of some
candidate local young galaxies. We present higher S/N data for these three
galaxies. UM151 and UM408 have significantly larger metallicities: log(O/H)+12
= 8.5 and 7.93, respectively. For A1228+12 our new log(O/H)+12 = 7.73 is close
to that recalculated from earlier data (7.68). Thus, the rederived
metallicities allow us to remove these objects from the list of galaxies with Z
< 1/20 Z_Sun.Comment: LaTeX, 8 pages with 3 Postscript figures, A&A in pres
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