128 research outputs found
A QSO survey via optical variability and zero proper motion in the M92 field.III. Narrow emission line galaxies
We study a sample of 23 narrow-emission line galaxies (NELGs) which were
selected by their strong variability as QSO candidates in the framework of a
variability-and-proper motion QSO survey on digitised Schmidt plates. In
previous work, we have shown that variability is an efficient method to find
AGNs. The variability properties of the NELGs are however significantly
different from those of the QSOs. The main aim of this paper is to clarify the
nature of this variability and to estimate the fraction of AGN-dominated NELGs
in this sample. New photometric and spectroscopic observations are presented,
along with revised data from the photographic photometry. The originally
measured high variability indices could not be confirmed. The diagnostic
line-ratios of the NELG spectra are consistent with HII region-like spectra. No
AGN could be proved, yet we cannot rule out the existence of faint
low-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry
We continue the QSO search in the 10 square degrees Schmidt field around M92
based on variability and proper motion (VPM) constraints. We have re-reduced
162 digitised B plates with a time-baseline of more than three decades and have
considerably improved both the photometric accuracy and the star-galaxy
separation at B>19. QSO candidates are selected and marked with one out of
three degrees of priority based on the statistical significance of their
measured variability and zero proper motion. Spectroscopic follow-up
observations of 84 new candidates with B>19 revealed an additional 37 QSOs and
7 Seyfert1s. In particular, all 92 high-priority candidates are
spectroscopically classified now; among them are 70 QSOs and 9 Seyfert1s
(success rate 86%). We expect that 87% (55%) of all QSOs with B<19.0 (19.8) are
contained in this high-priority subsample. For the combined sample of
high-priority and medium-priority objects, a completeness of 89% is estimated
up to B_lim=19.5. The sample of all AGNs detected in the framework of the VPM
search in the M92 field contains now 95 QSOs and 14 Seyfert1s with B<19.9.
Although the VPM QSOs were selected by completely different criteria, their
properties do not significantly differ from those of QSOs found by more
traditional optical survey techniques. In particular, the spectra and the
optical broad band colours do not provide any hints on a substantial population
of red QSOs up to the present survey limit.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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
Memberships and CM Diagrams of the Open Cluster NGC 7243
The results of astrometric and photometric investigations of the open cluster
NGC 7243 are presented. Proper motions of 2165 stars with root-mean-square
error of 1.1 mas/yr were obtained by means of PDS scanning of astrometric
plates covering the time interval of 97 years. A total of 211 cluster members
down to V=15.5 mag have been identified. V and B magnitudes have been
determined for 2118 and 2110 stars respectively. Estimations of mass (348Mo < M
< 522Mo), age (t=2.5x10^8 yr), distance (r=698 pc) and reddening (E(B-V)=0.24)
of the cluster NGC 7243 have been made.Comment: 24 pages, 15 figures. Submitted to Astronomy and Astrophysic
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
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
BVI Photometric Variability Survey of M3
We have conducted a three band (BVI) variability survey of the globular
cluster M3. This is the first three band survey of the cluster using modern
image subtraction techniques. Observations were made over 9 nights in 1998 on
the 1.2m telescope at the F.L. Whipple Observatory in Arizona. We present
photometry for 180 variable stars in the M3 field, of which 12 are newly
discovered. New discoveries include six SX Phe type variables which all lie in
the blue straggler region of the color magnitude diagram, two new first
overtone RR Lyrae, a candidate multi-mode RR Lyrae, a detached eclipsing
binary, and two unclassified variables. We also provide revised periods for 52
of the 168 previously known variables that we observe. The catalog and
photometry for the variable stars are available via anonymous ftp at
ftp://cfa-ftp.harvard.edu/pub/kstanek/M3/Comment: 35 pages, 11 figures. Movies associated with this paper at
http://www-cfa.harvard.edu/~jhartman/M3_movies.html Accepted for publication
in the Astronomical Journal. This replacement contains a few minor revision
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
An inverse method to interpret colour-magnitude diagrams
An inverse method is developed to determine the star formation history, the
age-metallicity relation, and the IMF slope from a colour-magnitude diagram.
The method is applied to the Hipparcos HR diagram. We found that the thin
disk of our Galaxy shows a peak of stellar formation 1.6 Gyr ago. The stars
close to the Sun have a solar metallicity and a mean IMF index equal to 3.2.
However, the model and the evolutionary tracks do not correctly reproduce the
horizontal giant branch.Comment: 14 pages, 11 figures. To be published in Astronomy & Astrophysic
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
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