43 research outputs found
New Galactic Candidate Luminous Blue Variables and Wolf-Rayet Stars
We have undertaken a near-infrared spectral survey of stars associated with
compact mid-IR shells recently revealed by the MIPSGAL (24 micron) and GLIMPSE
(8 micron) Spitzer surveys, whose morphologies are typical of circumstellar
shells produced by massive evolved stars. Through spectral similarity with
known Luminous Blue Variable (LBV) and Wolf-Rayet (WR) stars, a large
population of candidate LBVs (cLBVs) and a smaller number of new WR stars are
being discovered. This significantly increases the Galactic cLBV population and
confirms that nebulae are inherent to most (if not all) objects of this class.
Keywords - stars: emission-line, Be, stars: mass loss, stars: winds,
outflows, stars: Wolf-RayetComment: 2 pages, 1 figure. To appear in IAU 282 proceedings "From Interacting
Binaries to Exoplanets: Essential Modeling Tools", Mercedes Richards \& Ivan
Hubeny, ed
On the Nature of the Apparent Ring Galaxy SDSS J075234.33+292049.8
(abridged) An object classified as a galaxy in on-line data bases and
revealed on sky survey images as a distant ring galaxy is a rare case of polar
ring galaxy where the ring is only slightly inclined to the equatorial plane of
the central body. SDSS imaging indicates that the diameter of the ring is about
36 kpc. The SDSS data was combined with long-slit spectroscopic observations
and with Fabry-Perot Interferometer H-beta mapping obtained at the Russian
Academy of Sciences 6-m telescope. We derived the complex morphologies of this
presumed ring galaxy from a combination of SDSS images and from the kinematical
behaviour of the central body and of the ring, and determined the stellar
population compositions of the two components from SDSS colours, spectroscopy,
and evolutionary stellar synthesis models. The ring metallicity is slightly
under-abundant. The total luminosity and the total mass of the system are not
extreme, but the rather high M/L~20 indicates the presence of large amounts of
dark matter. Two alternative explanations of this object are proposed (1) a
ring formed by two semi-circular and tight spiral arms at the end of a central
bar with a warp or precession of the ring material. The object could,
therefore, be explained as an extreme SBa(R) galaxy, or (2) a Polar Ring Galaxy
where the inner object is an S0 and the ring is significantly more luminous
than the central object. The compound object would then be similar to the NGC
4650A galaxy, being a rare object with a polar component only modestly inclined
to the equatorial plane of the central body. Arguments for (and against) both
explanations are given and discussed, with the second alternative being more
acceptable.Comment: 16 pages, 11 figures, MNRAS in pres
Discovery of the first symbiotic star in NGC6822
We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in
the Local Group dwarf irregular galaxy NGC6822. This star was identified during
a spectral survey of Ha emission-line objects using the Southern African Large
Telescope (SALT) during its performance-verification phase. The observed strong
emission lines of HI and HeII suggest a high electron density and T* < 130 000
K for the hot companion. The infrared colours allow us to classify this object
as an S-type symbiotic star, comprising a red giant losing mass to a compact
companion. The red giant is an AGB carbon star, and a semi-regular variable,
pulsating in the first overtone with a period of 142 days. Its bolometric
magnitude is M_bol=-4.4 mag.
We review what is known about the luminosities of extragalactic symbiotic
stars, showing that most, possibly all, contain AGB stars. We suggest that a
much larger fraction of Galactic symbiotic stars may contain AGB stars than was
previously realised.Comment: 6 pages, 4 figures, accepted to MNRA
The Closest Known Flyby of a Star to the Solar System
Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 ("Scholz's star") to calculate its past trajectory. Integrating the Galactic orbits of this ~0.15 M_⊙ binary system and the Sun, we find that the binary passed within only 52^(+23)_(−14) kAU (0.25^(+0.11)_(−0.07) pc) of the Sun 70^(+15)_(−10) kya (1σ uncertainties), i.e., within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently (~0.1 Myr^(−1)). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is ~98%, but the probability of penetrating the dynamically active inner Oort Cloud (<20 kAU) is ~10^(−4). While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars
SBS 0335-052W - an Extremely Low Metallicity Dwarf Galaxy
We present Multiple Mirror Telescope (MMT) and Keck II telescope
spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western
component of the extremely low-metallicity blue compact galaxy SBS 0335-052.
The components, separated by 22 kpc, appear to be members of a unique,
physically connected system. It is shown that SBS 0335-052W consists of at
least three stellar clusters and has the same redshift as SBS 0335-052. The
oxygen abundance in its two brightest knots is extremely low, 12+log(O/H)=
7.22+/-0.03 and 7.13+/-0.08, respectively. These values are lower than in SBS
0335-052 and are nearly the same as those in I Zw 18. The (R-I) color profiles
are very blue in both galaxies due to the combined effects of ionized gas and a
young stellar population emission. We argue that SBS 0335-052W is likely to be
a nearby, young dwarf galaxy.Comment: 18 pages, 4 EPS figures, to appear in ApJ, 1 July 199
A Search for PNe in Nearby Galaxies with SDSS Imaging Data
We present the latest results from our project to search for new planetary
nebulae in nearby galaxies using Sloan Digital Sky Survey (SDSS) imaging data.
Our method is based on photometric criteria and can be applied to galaxies
where PNe appear as point sources. We applied these criteria to the whole area
of M31 as scanned by SDSS, detecting 130 new PN candidates and 30 known PNe.
All selected PNe candidates are located in the outer regions of M31. For 85
candidates follow-up spectroscopy was obtained with the 2.2m telescope at Calar
Alto Observatory. The observations show that our method has a detection
efficiency of about 82%. We discuss the 2D velocity field of the outer part of
M31 based on our observed PN data. The PNe suggest an exponential disk scale
length of 13 kpc along the minor axis. We discovered two PNe along the line of
sight to Andromeda NE, a very low surface brightness giant stellar structure in
the outer halo of M31. These two PNe are located at projected distances of ~48
kpc and ~41 kpc from the center of M31 and are the most distant PNe in M31
found up to now. Our data support the idea that Andromeda NE is located at the
distance of M31. No PNe associated with other M31 satellites observed by the
SDSS were found. Applying our method to other SDSS regions we checked data for
the Local Group galaxies Sextans, Draco, Leo I, Pegasus, Sextans B and Leo A
and recovered a known PN in Leo A. We re-measured its O/H and for the first
time determined abundances of N/H, S/H, He/H as well as the electron number
density Ne. We argue that the PN progenitor was formed ~1.5 Gyr ago during the
strongest episode of star formation in Leo A.Comment: 4 pages, 5 figures; contribution to proceedings of the conference
"Planetary Nebulae as Astronomical Tools
Discovery of Eight New Extremely Metal--Poor Galaxies in the Sloan Digital Sky Survey
We report the discovery of eight new extremely metal-poor galaxies (XMPGs;
12+log(O/H) < 7.65) and the recovery of four previously known or suspected
XMPGs (IZw18, HS0822+3542, HS0837+4717 and A1116+517) using Sloan Digital Sky
Survey (SDSS) spectroscopy. These new objects were identified after an analysis
of 250,000 galaxy spectra within an area of ~3000 deg^2 on the sky. Our oxygen
abundance determinations have an accuracy of 0.1 dex and are based on the
temperature-sensitive [O {\sc iii}] 4363 \AA line and on the direct
calculation of the electron temperature. We briefly discuss a new method of
oxygen abundance determinations using the [O {\sc ii}] 7319,7330 \AA\
lines, which is particularly useful for SDSS emission-line spectra with
redshifts ~0.024 since the [O {\sc ii}] 3727 \AA emission line
falls outside of the SDSS wavelength range. We detect XMPGs with redshifts
ranging from 0.0005 to 0.0443 and luminosities from 12\fm4 to
18\fm6. Our eight new XMPGs increase the number of known metal-deficient
galaxies by approximately one quarter. The estimated surface density of XMPGs
is 0.004 deg for 17\fm77.Comment: To appear in August 20 issue of ApJ Letters, 6 pages, 2 figure
Low-Surface-Brightness Galaxies in the Sloan Digital Sky Survey. I. Search Method and Test Sample
In this paper we present results of a pilot study to use imaging data from
the Sloan Digital Sky Survey (SDSS) to search for low-surface-brightness (LSB)
galaxies. For our pilot study we use a test sample of 92 galaxies from the
catalog of Impey et al. (1996) distributed over 93 SDSS fields of the Early
Data Release (EDR). Many galaxies from the test sample are either LSB or dwarf
galaxies. To deal with the SDSS data most effectively a new photometry software
was created, which is described in this paper. We present the results of the
selection algorithms applied to these 93 EDR fields. Two galaxies from the
Impey et al. test sample are very likely artifacts, as confirmed by follow-up
imaging. With our algorithms, we were able to recover 87 of the 90 remaining
test sample galaxies, implying a detection rate of 96.5%. The three
missed galaxies fall too close to very bright stars or galaxies. In addition,
42 new galaxies with parameters similar to the test sample objects were found
in these EDR fields (i.e., 47% additional galaxies). We present the main
photometric parameters of all identified galaxies and carry out first
statistical comparisons. We tested the quality of our photometry by comparing
the magnitudes for our test sample galaxies and other bright galaxies with
values from the literature. All these tests yielded consistent results. We
briefly discuss a few unusual galaxies found in our pilot study, including an
LSB galaxy with a two-component disk and ten new giant LSB galaxies.Comment: 36 pages, 16 figures, accepted for publication by AJ, some figures
were bitmapped to reduce the siz