194 research outputs found
The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe
In this work we present the first spectroscopic results obtained with the
Southern African Large Telescope (SALT) telescope during its
perfomance-verification phase. We find that the Sagittarius dwarf spheroidal
galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age
t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on
spectra of two planetary nebulae (PNe), using empirical abundance
determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He.
We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02
dex. The other PN studied, BoBn1, is an extraordinary object in that the neon
abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield
the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16
dex which is 1/110 of the solar value. The actual [O/H] is much higher: third
dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen
and ~70 in neon. Neon as well as nitrogen and oxygen content may have been
produced in the intershell of low-mass AGB stars. Well defined broad WR lines
are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.Comment: 14 pages, 4 figures, accepted to MNRA
Near and mid-infrared colours of star-forming galaxies in ELAIS fields
We present J and K-band near-infrared photometry of a sample of mid-infrared
sources detected by the Infrared Space Observatory (ISO) as part of the
European Large Area ISO-Survey (ELAIS) and study their classification and
star-forming properties. We have used the Preliminary ELAIS Catalogue for the
6.7 micron (LW2) and 15 micron (LW3) fluxes. All of the high-reliability LW2
sources and 80 per cent of the LW3 sources are identified in the near-IR survey
reaching K = 17.5 mag. The near- to mid-IR flux ratios can effectively be used
to separate stars from galaxies in mid-IR surveys. The stars detected in our
survey region are used to derive a new accurate calibration for the ELAIS
ISOCAM data in both the LW2 and LW3 filters. We show that near to mid-IR
colour-colour diagrams can be used to further classify galaxies, as well as
study star-formation. The ISOCAM ELAIS survey is found to mostly detect
strongly star-forming late-type galaxies, possibly starburst powered galaxies,
and it also picks out obscured AGN. The ELAIS galaxies yield an average mid-IR
flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss this [6.7/15] ratio as a star
formation tracer using ISO and IRAS data of a local comparison sample. We find
that the [K/15] ratio is also agood indicator of activity level in galaxies and
conclude that the drop in the [6.7/15] ratio seen in strongly star-forming
galaxies is a result of both an increase of 15 mic emission and an apparent
depletion of 6.7 mic emission. Near-IR data together with the mid-IR give the
possibility to estimate the relative amount of interstellar matter in the
galaxies.Comment: 18 pages, 15 figures; accepted for publication in MNRA
Intra-orogenic Svecofennian magmatism in SW Finland constrained by LA-MC-ICP-MS zircon dating and geochemistry
We have studied plutonic rocks from the Korpo and Rauma areas of south-western Finland which can be categorized as intra-orogenic, i.e. they were intruded during a proposed extensional period between the two main Svecofennian orogenic cycles: the Fennian and Svecobaltic orogenies. The diorite from Rauma yielded an age of 1865 +/- 9 Ma and the diorite from Korpo an age of 1852 +/- 4 Ma. The adjacent garnet-bearing Korpo granite was 1849 +/- 8 Ma in age. Zircons from the granite also included inherited Archaean and older Palaeoproterozic zircons, as well as metamorphic c. 1820 Ma rims. The diorites are high-K to shoshonitic, mantle-derived magmas, rich in Fe, P, F and light rare earth elements. The Korpo granites show typical features of crustal-derived melts and form hybrids with the diorites in contact zones. Both the mantle-derived and crustal-derived intra-orogenic magmatism are considered to have had a causal effect on the subsequent late Svecofennian (Svecobaltic) thermal evolution in southern Finland which culminated in granulite facies metamorphism and large-scale crustal melting.</p
V5852 Sgr : an unusual nova possibly associated with the Sagittarius stream
We report spectroscopic and photometric follow-up of the peculiar nova V5852~Sgr (discovered as OGLE-2015-NOVA-01), which exhibits a combination of features from different nova classes. The photometry shows a flat-topped light curve with quasi-periodic oscillations, then a smooth decline followed by two fainter recoveries in brightness. Spectroscopy with the Southern African Large Telescope shows first a classical nova with an Fe II or Fe IIb spectral type. In the later spectrum, broad emissions from helium, nitrogen and oxygen are prominent and the iron has faded which could be an indication to the start of the nebular phase. The line widths suggest ejection velocities around 1000 km s-1. The nova is in the direction of the Galactic bulge and is heavily reddened by an uncertain amount. The V magnitude 16 days after maximum enables a distance to be estimated and this suggests that the nova may be in the extreme trailing stream of the Sagittarius dwarf spheroidal galaxy. If so it is the first nova to be detected from that, or from any dwarf spheroidal galaxy. Given the uncertainty of the method and the unusual light curve we cannot rule out the possibility that it is in the bulge or even the Galactic disk behind the bulge.Publisher PDFPeer reviewe
The absence of sub-minute periodicity in classical T Tauri stars
Classical T Tauri stars (CTTS) are young, late-type objects, that still
accrete matter from a circumstellar disk. Analytical treatments and numerical
simulations predict instabilities of the accretion shock on the stellar
surface. We search for variability on timescales below a few minutes in the
CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern
African Large Telescope (SALT) in narrow-band filters around the Balmer jump.
The observations were performed in slit mode, which provides a time resolution
of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single
photon-counting device with even better time resolution. Small-scale
variability typically lasts a few seconds, however, no significant periodicity
is detected. We place a 99 % confidence upper limit on the pulsed fraction of
the lightcurves. The relative amplitude is below 0.001 for TW Hya in the
frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm
filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50
Hz. The relevant timescales indicate that shock instabilites should not be seen
directly in our optical and UV observations, but the predicted oscialltions
would induce observable variations in the reddening. We discuss how the
magnetic field could stabilise the accretion shock.Comment: 8 pages, 9 figures, accepted by A&
The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Our goal is to characterize AGN populations by comparing their X-ray and
optical classifications. We present a sample of 99 spectroscopically identified
X-ray point sources in the XMM-LSS survey which are significantly detected in
the [2-10] keV band, and with more than 80 counts. We performed an X-ray
spectral analysis for all of these 99 X-ray sources. Introducing the fourfold
point correlation coefficient, we find only a mild correlation between the
X-ray and the optical classifications, as up to 30% of the sources have
differing X-ray and optical classifications: on one hand, 10% of the type 1
sources present broad emission lines in their optical spectra and strong
absorption in the X-rays. These objects are highly luminous AGN lying at high
redshift and thus dilution effects are totally ruled out, their discrepant
nature being an intrinsic property. Their X-ray luminosities and redshifts
distributions are consistent with those of the unabsorbed X-ray sources with
broad emission lines. On the other hand, 25/32 are moderate luminosity AGN,
which are both unabsorbed in the X-rays and only present narrow emission lines
in their optical spectra. The majority of them have an optical spectrum which
is representative of the host galaxy. We finally infer that dilution of the AGN
by the host galaxy seems to account for their nature. 5/25 have been defined as
Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted
for by the standard AGN unified scheme, as its predictions are not met for only
12% of the 99 X-ray sources. ABRIDGEDComment: 25 pages, 19 figures, Accepted for publication in A&
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