84 research outputs found
The reddest ISO-2MASS quasar
In the course of the NIR/MIR AGN search combining the 6.7 mu ISOCAM Parallel
Survey and 2MASS we have discovered 24 type-1 quasars about a third of which
are too red to be discriminated by optical/UV search techniques. Here we report
on a detailed case study of the reddest type-1 quasar of our sample (J2341) at
redshift z=0.236 with M_K=-25.8 and J-K=1.95. We performed spectroscopy in the
optical with VLT/FORS1 and in the MIR with Spitzer as well as NIR imaging with
ISPI at CTIO. The optical and NIR observations reveal a star forming
emission-line galaxy at the same redshift as the quasar with a projected linear
separation of 1.8 arcsec (6.7 kpc). The quasar and its companion are embedded
in diffuse extended continuum emission. Compared with its companion the quasar
exhibits redder optical-NIR colours, which we attribute to hot nuclear dust.
The MIR spectrum shows only few emission lines superimposed on a power-law
spectral energy distribution. However, the lack of strong FIR emission suggests
that our potentially interacting object contains much less gas and dust and is
in a stage different from dust reddened ULIRG-AGN like Mrk 231. The optical
spectrum shows signatures for reddening in the emission-lines and no
post-starburst stellar population is detected in the host galaxy of the quasar.
The optical continuum emission of the active nucleus appears absorbed and
diluted. Even the combination of absorption and host dilution is not able to
match J2341 with standard quasar templates. While the BLR shows only a rather
moderate absorption of E_(B-V)=0.3, the continuum shorter than 4500 AA requires
strong obscuration with E_(B-V)=0.7, exceeding the constraints from the low
upper limit on the 9.7 mu silicate absorption. This leads us to conclude that
the continuum of J2341 is intrinsically redder than that of typical quasars.Comment: 8 pages, 9 figure
UKIRT follow-up observations of the old open cluster FSR0358 (Kirkpatrick1)
We aim to characterise the properties of the stellar clusters in the Milky
Way. Utilising an expectation-maximisation method we determined that the
cluster FSR0358, originally discovered by J.D.Kirkpatrick, is the most likely
real cluster amongst the cluster candidates from Froebrich et al.. Here we
present new deep high resolution near infrared imaging of this object obtained
with UKIRT. The analysis of the data reveals that FSR0358 (Kirkpatrick1) is a
5+-2Gyr old open cluster in the outer Milky Way. Its age, metallicity of
Z=0.008 and distance from the Galactic Centre of 11.2kpc are typical for the
known old open galactic clusters. So far six of the FSR cluster candidates have
been identified as having an age above 5Gyr. This shows the significance of
this catalogue in enhancing our knowledge of the oldest open clusters in the
Galaxy.Comment: 8 pages, 1 table, 5 figures, accepted for publication by MNRAS, a
version with higher resolution figures can be found at
http://astro.kent.ac.uk/~df
Mid-infrared selection of AGN
Since a large fraction of active galactic nuclei (AGN) is missed in common
UV-excess surveys and is even hard to find in radio, near-IR and X-ray surveys,
we have used a new AGN selection technique which is expected to be not affected
by extinction. Within the scientific verification of the ISOCAM Parallel Survey
at 6.7 micron we have discovered objects with exceptional mid-infrared (MIR)
emission. They are essentially not detected on IRAS-ADDSCANs and only very few
of them show up in the NVSS and FIRST radio surveys. Various colour criteria of
the 6.7 micron data with 2MASS and optical wavebands show that the sources
reach more extreme IR colours than the sources in the Hubble Deep Field-South
and the ELAIS survey. The comparison with known object types suggests that we
have found AGN with a pronounced MIR emission, probably due to circum-nuclear
dust. First results from optical spectroscopy of ten candidates corroborate
this interpretation showing four AGN, two reddened LINER and four extremely
reddened emission-line galaxies with MIR/FIR flux ratios higher than for known
pure starburst galaxies. The results will make a significant contribution to
the debate on the entire AGN population.Comment: 4 pages, 2 figures, accepted for publication as Letter in Astronomy &
Astrophysic
Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster
We present near-infrared photometry and optical spectroscopy of very low-mass
stars and brown dwarf candidates in the Pleiades open cluster. The membership
status of these objects is assessed. Eight objects out of 45 appear to be
non-members. A search for companions among 34 very low-mass Pleiades members
(M0.09 M) in high-spatial resolution images obtained with the
Hubble Space Telescope and the adaptive optics system of the
Canada-France-Hawaii telescope produced no resolved binaries with separations
larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find
evidence for a binary sequence in the color-magnitude diagrams, in agreement
with the results of Steele & Jameson (1995) for higher mass stars. We compare
the multiplicity statistics of the Pleiades very low-mass stars and brown
dwarfs with that of G and K-type main sequence stars in the solar neighborhood
(Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency
of wide binary systems (separation >27 AU) among the Pleiades very low-mass
members. We briefly discuss how this result can fit with current scenarios of
brown dwarf formation. We correct the Pleiades substellar mass function for the
contamination of cluster non-members found in this work. We find a
contamination level of 33% among the brown dwarf candidates identified by
Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary,
we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass
bin is still rising but the contribution to the total mass of the cluster is
declining in the brown dwarf regime.Comment: to be published in The Astrophysical Journa
Contribution of White Dwarfs to Cluster Masses
I present a literature search through 31 July 1997 of white dwarfs (WDs) in
open and globular clusters. There are 36 single WDs and 5 WDs in binaries known
among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among
11 globular clusters. From these data I have calculated WD mass fractions for
four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one
globular cluster (NGC 6121). I develop a simple model of cluster evolution that
incorporates stellar evolution but not dynamical evolution to interpret the WD
mass fractions. I augment the results of my simple model with N-body
simulations incorporating stellar evolution (Terlevich 1987; de la Feunte
Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters
undergo moderate to strong kinematical evolution the WD mass fraction is
relatively insensitive to kinematical evolution. By comparing the cluster mass
functions to that of the Galactic disk, and incorporating plausibility
arguments for the mass function of the Galactic halo, I estimate the WD mass
fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old
(Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that
the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et
al. 1998), although the WD mass fraction is insensitive to age in this range. I
find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps
as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The
Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent
with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et
al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be
published in AJ, April, 199
Stellar Pollution in the Solar Neighborhood
We study spectroscopically determined iron abundances of 642 solar-type stars
to search for the signature of accreted iron-rich material. We find that the
metallicity [Fe/H] of a subset of 466 main sequence stars, when plotted as a
function of stellar mass, mimics the pattern seen in lithium abundances in open
clusters. Using Monte Carlo models we find that, on average, these stars have
accreted about 0.4 Earth masses of iron while on the main sequence. A much
smaller sample of 19 stars in the Hertzsprung gap, which are slightly evolved
and whose convection zones are significantly more massive, have lower average
[Fe/H], and their metallicity shows no clear variation with stellar mass. These
findings suggest that terrestrial-type material is common around solar type
stars.Comment: 33 pages, 11 figures. Submitted to Ap
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
An isotopic analysis of ionising radiation as a source of sulphuric acid
Sulphuric acid is an important factor in aerosol nucleation and growth. It has been shown that ions enhance the formation of sulphuric acid aerosols, but the exact mechanism has remained undetermined. Furthermore some studies have found a deficiency in the sulphuric acid budget, suggesting a missing source. In this study the production of sulphuric acid from SO<sub>2</sub> through a number of different pathways is investigated. The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays. The distributions of stable sulphur isotopes in the products and substrate were measured using isotope ratio mass spectrometry. All methods produced sulphate enriched in <sup>34</sup>S and we find an enrichment factor (&delta;<sup>34</sup>S) of 8.7 ± 0.4&permil; (1 standard deviation) for the UV-initiated OH reaction. Only UV light (Hg emission at 253.65 nm) produced a clear non-mass-dependent excess of <sup>33</sup>S. The pattern of isotopic enrichment produced by gamma rays is similar, but not equal, to that produced by aqueous oxidation of SO<sub>2</sub> by ozone. This, combined with the relative yields of the experiments, suggests a mechanism in which ionising radiation may lead to hydrated ion clusters that serve as nanoreactors for S(IV) to S(VI) conversion
The History of the Mysterious Eclipses of KH 15D II. Asiago, Kiso, Kitt Peak, Mt. Wilson, Palomar, Tautenburg and Rozhen Observatories, 1954-97
The unusual pre-main-sequence binary star named KH 15D undergoes remarkably
deep and long-lasting periodic eclipses. Some clues about the reason for these
eclipses have come from the observed evolution of the system's light curve over
the last century. Here we present UBVRI photometry of KH 15D based on
photographic plates from various observatories, ranging in time from 1954 to
1997. The system has been variable at the ~1 mag level since at least 1965.
There is no evidence for color variations, with a typical limit of Delta(B-V) <
0.2 mag. We confirm some previously published results that were based on a
smaller sample of plates: from approximately 1965 to 1990, the total flux was
modulated with the 48-day orbital period of the binary, but the maximum flux
was larger, the fractional variations were smaller, and the phase of minimum
flux was shifted by almost a half-cycle relative to the modern light curve. All
these results are consistent with the recently proposed theory that KH 15D is
being occulted by an inclined, precessing, circumbinary ring.Comment: 10 pages, 3 figures, 3 tables, accepted for publication in
Astronomical Journa
- …