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 (M$\le$0.09 M$_\odot$) 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 ($\sim 1$ 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 (δ<sup>34</sup>S) of 8.7 ± 0.4‰ (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