Chemical evolution and depletion pattern in Damped Lyman-alpha (DLA) systems

In this paper we point out a previously unnoticed anticorrelation between the observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X stands for the refractories Fe, Cr and Ni) and metal column density ([Zn/H]+log(N{HI})) in DLAs. We suggest that this trend is an unambiguous sign of dust depletion, since metal column density is a measure of the amount of dust along the line of sight. Assuming that DLAs are (proto-)galactic disks and using detailed chemical evolution models with metallicity dependent yields we study chemical evolution and dust depletion patterns for alpha and iron-peak elements in DLAs. When observational constraints on the metal column density of DLAs are taken into account (as suggested in Boisse et al. 1998) we find that our models reproduce fairly well the observed mild redshift evolution of the abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the observed scatter at a given redshift. By considering the aforementioned dependence of abundance ratios on metal column density, we further explore the general dust depletion pattern in DLAs, comparing to our model results and to a solar reference pattern. We suggest that further measurements of the key elements, i.e. Zn, S and Mn, will help to gain more insight into the nature of DLAs. In any case, the presently uncertain nucleosynthesis of Zn in massive stars (on which a large part of these conclusions is based) should be carefully scrutinised.Comment: 12 pages, 4 figures, Astronomy and Astrophysics, in pres

No compelling evidence of distributed production of CO in comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a reanalysis of near-IR lines

Based on long-slit infrared spectroscopic observations, it has been suggested that half of the carbon monoxide present in the atmosphere of comet C/1995 O1 (Hale-Bopp) close to perihelion was released by a distributed source in the coma, whose nature (dust or gas) remains unidentified. We re-assess the origin of CO in Hale-Bopp's coma from millimeter interferometric data and a re-analysis of the IR lines. Simultaneous observations of the CO J(1-0) (115 GHz) and J(2-1) (230 GHz) lines were undertaken with the IRAM interferometer in single-dish and interferometric modes. The diversity of angular resolutions (from 1700 to 42000 km diameter at the comet) is suitable to study the radial distribution of CO and detect the extended source observed in the infrared. We used excitation and radiative transfer models to simulate the observations. Various CO density distributions were considered, including 3D time-dependent hydrodynamical simulations which reproduce a CO rotating jet. The CO J(1-0) and J(2-1) observations can be consistently explained by a nuclear production of CO. Composite 50:50 nuclear/extended productions with characteristic scale lengths of CO parent L_p > 1500 km are rejected. Based on similar radiation transfer calculations, we show that the CO v = 1-0 ro-vibrational lines observed in comet Hale-Bopp at heliocentric distances less than 1.5 AU are severely optically thick. The broad extent of the CO brightness distribution in the infrared is mainly due to optical depth effects. Additional factors can be found in the complex structure of the CO coma, and non-ideal slit positioning caused by the anisotropy of dust IR emission. We conclude that both CO millimeter and infrared lines do not provide compelling evidence for a distributed source of CO in Hale-Bopp's atmosphere.Comment: Accepted for publication in Icarus (55 pages, 13 figures

Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst

Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the sudden fragmentation of its nucleus and the production of a large quantity of grains scattering sunlight. We report on 90 GHz continuum observations carried out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007 UT, i.e., 4-5 days after the outburst. These observations probed the thermal radiation of large dust particles, and therefore provide the best constraints on the mass in the ejecta debris. The thermal emission of the debris was modelled and coupled to a time-dependent description of their expansion after the outburst. The analysis was performed in the Fourier plane. Visibilities were computed for the two observing dates and compared to the data to measure their velocity and mass. Optical data and 250-GHz continuum measurements published in the literature were used to further constrain the dust kinematics and size distribution. Two distinct dust components in terms of kinematic properties are identified in the data. The large-velocity component, with typical velocities V0 of 50-100 m/s for 1 mm particles, displays a steep size distribution with a size index estimated to q = -3.7 (\pm0.1), assuming a minimum grain size of 0.1 \mum. It corresponds to the fast expanding shell observed in optical images. The slowly-moving "core" component (V0 = 7-9 m/s) detected near the nucleus has a size index |q| < 3.4 and contains a higher proportion of large particles than the shell. The dust mass in the core is in the range 0.1-1 that of the shell. Using optical constants pertaining to porous grains (50% porosity) made of astronomical silicates mixed with water ice (48% in mass), the total dust mass Mdust injected by the outburst is estimated to 4-14 x 10**11 kg, corresponding to 3-9% the nucleus mass.Comment: 15 pages with 11 figures and 7 tables. Accepted for publication in Astronomy & Astrophysic

Star formation history of galaxies from z=0 to z=0.7 A backward approach to the evolution of star-forming galaxies

We investigate whether the mean star formation activity of star-forming galaxies from z=0 to z=0.7 in the GOODS-S field can be reproduced by simple evolution models of these systems. In this case, such models might be used as first order references for studies at higher z to decipher when and to what extent a secular evolution is sufficient to explain the star formation history in galaxies. We selected star-forming galaxies at z=0 and at z=0.7 in IR and in UV to have access to all the recent star formation. We focused on galaxies with a stellar mass ranging between 10^{10} and 10^{11} M_sun for which the results are not biased by the selections. We compared the data to chemical evolution models developed for spiral galaxies and originally built to reproduce the main characteristics of the Milky Way and nearby spirals without fine-tuning them for the present analysis. We find a shallow decrease in the specific star formation rate (SSFR) when the stellar mass increases. The evolution of the SSFR characterizing both UV and IR selected galaxies from z=0 to z=0.7 is consistent with the models built to reproduce the present spiral galaxies. There is no need to strongly modify of the physical conditions in galaxies to explain the average evolution of their star formation from z=0 to z=0.7. We use the models to predict the evolution of the star formation rate and the metallicity on a wider range of redshift and we compare these predictions with the results of semi-analytical models.Comment: 14 pages, 10 figures. accepted for publication in Astronomy & Astrophysic

From Spirals to Low Surface Brightness galaxies

We show that simple models of the chemical and spectrophotometric evolution of galaxies can be used to explore the properties of present-day galaxies and especially the causes of the observed variety among disc galaxies. We focus on the link between ``classical'' spirals and Low Surface Brightness galaxies.Comment: 4 pages, To appear in the Proceedings of the Euroconference The Evolution of Galaxies: III. From simple approaches to self-consistent models (Kluwer

Abundance gradients and their evolution in the Milky Way disk

Based on a simple, but fairly successful, model of the chemical evolution of the Milky Way disk, we study the evolution of the abundances of the elements He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields for massive stars with and without mass loss. We find that most observed abundance profiles are correctly reproduced by massive star yields, but C and N require supplementary sources. We argue that massive, mass losing stars can totally account for the abundance profile of C, while intermediate mass stars are the main source of N; in both cases, some conflict with corresponding data on extragalactic HII regions arises, at least if current observations in the Galaxy are taken at face value. The observed behaviour of Al is marginally compatible with current massive star yields, which probably overestimate the ``odd-even'' effect. We also find that the adopted ``inside-out'' formation scheme for the Milky Way disk produce abundance profiles steeper in the past. The corresponding abundance scatter is smaller in the inner disk than in the outer regions for a given interval of Galactic age

Millimetre continuum observations of comet C/2009 P1 (Garradd)

Little is known about the physical properties of the nuclei of Oort cloud comets. Measuring the thermal emission of a nucleus is one of the few means for deriving its size and constraining some of its thermal properties. We attempted to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We used the Plateau de Bure Interferometer to measure the millimetric thermal emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the observations at 266 GHz were not usable due to bad atmospheric conditions, we derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at 157 GHz. Using a thermal model for a spherical nucleus with standard thermal parameters, we found an upper limit of 5.6 km for the radius. The dust contribution to our signal is estimated to be negligible. Given the water production rates measured for this comet and our upper limit, we estimated that Garradd was very active, with an active fraction of its nucleus larger than 50%.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 2 figure

The properties of the Malin 1 galaxy giant disk: A panchromatic view from the NGVS and GUViCS surveys

Low surface brightness galaxies (LSBGs) represent a significant percentage of local galaxies but their formation and evolution remain elusive. They may hold crucial information for our understanding of many key issues (i.e., census of baryonic and dark matter, star formation in the low density regime, mass function). The most massive examples - the so called giant LSBGs - can be as massive as the Milky Way, but with this mass being distributed in a much larger disk. Malin 1 is an iconic giant LSBG, perhaps the largest disk galaxy known. We attempt to bring new insights on its structure and evolution on the basis of new images covering a wide range in wavelength. We have computed surface brightness profiles (and average surface brightnesses in 16 regions of interest), in six photometric bands (FUV, NUV, u, g, i, z). We compared these data to various models, testing a variety of assumptions concerning the formation and evolution of Malin 1. We find that the surface brightness and color profiles can be reproduced by a long and quiet star-formation history due to the low surface density; no significant event, such as a collision, is necessary. Such quiet star formation across the giant disk is obtained in a disk model calibrated for the Milky Way, but with an angular momentum approximately 20 times larger. Signs of small variations of the star-formation history are indicated by the diversity of ages found when different regions within the galaxy are intercompared.For the first time, panchromatic images of Malin 1 are used to constrain the stellar populations and the history of this iconic example among giant LSBGs. Based on our model, the extreme disk of Malin 1 is found to have a long history of relatively low star formation (about 2 Msun/yr). Our model allows us to make predictions on its stellar mass and metallicity.Comment: Accepted in Astronomy and Astrophysic