arXiv subjects

A. Weiss

Publications and source records attributed to A. Weiss.

At least 199 records · Page 11Linked to original sources

arXiv2004

Atomic carbon at redshift ~2.5

Using the IRAM 30m telescope we detected the lower fine structure line of neutral carbon towards three high--redshift sources: IRAS FSC10214 (z=2.3), SMMJ14011+0252 (z=2.5) and H1413+117 (Cloverleaf quasar, z=2.5). SMMJ14011+0252 is the first high--redshift, non--AGN source in which CI has been detected. The CI(1-0) line from FSC10214 is almost an order of magnitude weaker than previously claimed, while our detection in the Cloverleaf is in good agreement with earlier observations. The CI(1-0) linewidths are similar to the CO widths, indicating that both lines trace similar regions of molecular gas on galactic scales. Derived CI masses for all three objects are of order few 10^7 solar masses and the implied CI(1-0)/CO(3-2) line luminosity ratio is about 0.2. This number is similar to values found in local galaxies. We derive a CI abundance of 5x10^{-5} which implies significant metal enrichment of the cold molecular gas at redshifts 2.5 (age of the universe 2.7 Gyr). We conclude that the physical properties of systems at large lookback times are similar to today's starburst/AGN environments.

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arXiv2004

Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF

We investigate the bright cut-off of the [OIII]l5007 planetary nebula luminosity function (PNLF), that has been suggested as a powerful extragalactic distance indicator. Theoretical PNLFs are constructed via Monte-Carlo simulations of populations of PNe, whose individual properties are described with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a detailed photoionisation code (CLOUDY). The basic dependences of the cut-off magnitude M* are then discussed. We find that: (i) In galaxies with recent or ongoing star formation, the modelled PNLF present M* values between -4 and -5, very close to the observationally-calibrated value for the LMC. (ii) In these galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about 2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence M* is expected to depend strongly on the age of the last burst of star formation, dimming by as much as 5 mag as we go from young to 10-Gyr old populations. (iii) Rather than on the initial metallicity of a stellar population, M* depends on the actual [O/H] of the observed PNe, a quantity that may differ significantly from the initial value (due to dredge-up episodes), especially in young and intermediate-age PN populations. (iv) Also the transition time from the end of AGB to the PN phase, and the nuclear-burning properties (i.e. H- or He-burning) of the central stars introduce non-negligible effects on M*. The strongest indication derived from the present calculations is a serious difficulty to explain the age-invariance of the cut-off brightness over an extended interval, say from 1 to 13 Gyr, that observations of PNLFs in galaxies of late-to-early type seem to suggest.

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arXiv2004

Models for extremely metal-poor halo stars

Two alternative scenarios concerning the origin and evolution of extremely metal-poor halo stars are investigated. The first one assumes that the stars have been completely metal-free initially and produced observed carbon and nitrogen overabundances during the peculiar core helium flash typical for low-mass Population-III stars. The second scenario assumes that the initial composition resulted from a mixture of primordial material with ejecta from a single primordial supernovae. Both scenarios are shown to have problems in reproducing C, N, and O abundances simultaneously, and both disagree with observed 12C/13C-ratios,though in different directions. We concentrate on the most iron-poor, carbon-rich object of this class, HE0107-5240, and conclude that the second scenario presently offers the more promising approach to understand these objects, in particular because evolutionary tracks match observations very well.

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arXiv2003

Gas and Dust in the Cloverleaf Quasar at Redshift 2.5

We observed the upper fine structure line of neutral carbon, CI(2-1), the CO(3-2) line and the 1.2mm continuum emission from H1413+117 (Cloverleaf quasar, z=2.5) using the IRAM interferometer. Together with the detection of the lower fine structure line (Barvainis etal. 1997), the Cloverleaf quasar is now only the second extragalactic system, besides M82, where both carbon lines have convincingly been detected. Our analysis shows that the carbon lines are optically thin and have an excitation temperature of ~30 K. CO is subthermally excited and the observed line luminosity ratios are consistent with n(H2)=10^(3-4) cm^(-3) at Tkin=30-50 K. Using three independent methods (CI, dust, CO) we derive a total molecular gas mass (corrected for magnification) of M(H2)=1.2+/-0.3*10^(10) SM. Our observations suggest that the molecular disk extends beyond the region seen in CO(7-6) to a zone of more moderately excited molecular gas that dominates the global emission in CI and the low J CO lines.

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arXiv2003

Spectroscopic Study of Blue Compact Galaxies II. Spectral Analysis and Correlations

This is the second paper in a series studying the star formation rates, stellar components, metallicities, and star formation histories and evolution of a sample of blue compact galaxies. We analyzed spectral properties of 97 blue compact galaxies, obtained with the Beijing Astronomical Observatory (China) 2.16 m telescope, with spectral range 3580Å-- 7400Å. We classify the spectra according to their emission lines: 13 of the total 97 BCGs sample are non-emission line galaxies (non-ELGs); 10 have AGN-like emission (AGNs), and 74 of them are star-forming galaxies (SFGs). Emission line fluxes and equivalent widths, continuum fluxes, the 4000 ÅBalmer break index and equivalent widths of absorption lines are measured from the spectra. We investigate the emission line trends in the integrated spectra of the star-forming galaxies in our sample, and find that: 1) The equivalent widths of emission lines are correlated with the galaxy absolute blue magnitude $M_B$; lower luminosity systems tend to have larger equivalent widths. 2) The equivalent width ratio [NII]6583/H$α$ is anti-correlated with equivalent width H$α$; a relationship is given that can be used to remove the [NII] contribution from blended H$α$ + [NII]6548, 6583. 3) The [OII], H$β$, H$γ$ and H$α$ fluxes are correlated; those can be used as star formation tracers in the blue. 4) The metallicity indices show trends with galaxy absolute magnitude and attenuation by dust; faint, low-mass BCGs have lower metallicity and color excess.

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arXiv2003

Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342

The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342. The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9) transition) above the ground state and reveal a warm (T_kin= 100 - 140 K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342. Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are 1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The temperatures and NH3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M82, however, the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense (to provide sufficient NH3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H_2 in M82 and the observed chemical abundances.

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arXiv2002

First full evolutionary computation of the He-flash induced mixing in Population II stars

The core helium-flash in low-mass stars with extreme mass loss occurs after the tip of the RGB, when the H-rich envelope is very thin. The low efficiency of the H-shell source enables the He-flash driven convective zone to penetrate H-rich layers and trigger a thermonuclear runaway, resulting in a subsequent surface enrichment with He and C. In this work we present the first full computations of Population II low-mass stellar models through this phase. Models experiencing this dredge-up event are significantly hotter than their counterparts with H-rich envelopes, which makes them promising candidates for explaining the existence of stars observed beyond the canonical blue end of the horizontal branch ("blue hook stars"). Moreover, this temperature difference could explain the observed gap in M_V between extreme blue horizontal-branch and blue hook stars. A first comparison with spectroscopic observations of blue hook stars in the globular cluster omega Cen is also presented.

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arXiv2002

The surface carbon and nitrogen abundances in models of ultra metal-poor stars

We investigate whether the observed high number of carbon- and nitrogen-enhanced extremely metal-poor stars could be explained by peculiar evolutionary properties during the core He flash at the tip of the red giant branch. For this purpose we compute a series of detailed stellar models expanding upon our previous work; in particular, we investigate if during the major He flash the penetration of the helium convective zone into the overlying hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in agreement with current spectroscopic observations. The dependence of this phenomenon on selected model input parameters, such as initial metallicity and treatment of convection is examined in detail.

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arXiv2002

Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets

We provide tables of theoretical isochrones in several photometric systems. To this aim, the following steps are followed: (1) First, we re-write the formalism for converting synthetic stellar spectra into tables of bolometric corrections. The resulting formulas can be applied to any photometric system, provided that the zero-points are specified by means of either ABmag, STmag, VEGAmag, or a standard star system that includes well-known spectrophotometric standards. Interstellar absorption can be considered in a self-consistent way. (2) We assemble an extended and updated library of stellar intrinsic spectra. It is mostly based on non-overshooting ATLAS9 models, suitably extended to both low and high effective temperatures. This offers an excellent coverage of the parameter space of Teff, logg, and [M/H]. We briefly discuss the main uncertainties and points still deserving more improvement. (3) From the spectral library, we derive tables of bolometric corrections for Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey systems (this latter consisting on the WFI, EMMI, and SOFI filter sets). (4) These tables are used to convert several sets of Padova isochrones into the corresponding absolute magnitudes and colours, thus providing a useful database for several astrophysical applications. All data files are made available in electronic form.

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arXiv2002

Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms

We present homogeneous age determinations for a large sample of 55 Galactic globular clusters, which constitute about 30% of the total Galactic population. A study of their age distribution reveals that all clusters from the most metal poor ones up to intermediate metallicities are coeval, whereas at higher [Fe/H] an age spread exists, together with an age-metallicity relationship. At the same time, all clusters within a certain galactocentric distance appear coeval, whereas an age spread is present further away from the Galactic centre, without any correlation with distance. The precise value of [Fe/H] and galactocentric distance for the onset of the age spread and the slope of the age-metallicity relationship are strongly affected by the as yet uncertain [Fe/H] scale. We discuss how differences in the adopted [Fe/H] scale and cluster sample size may explain discrepant results about the clusters age distribution reached by different authors. Taking advantage of the large number of objects included in our sample, we also tested the possibility that age is the global second parameter which determines the Horizontal Branch morphology, and found indications that age could explain the global behaviour of the second parameter effect.

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arXiv2002

Molecular gas in NGC6946

We present imaging of molecular gas emission in the star-forming spiral galaxy NGC6946. Our CO(1-0) and CO(3-2) images, made at 22" resolution with the IRAM 30-m and the Heinrich Hertz 10-m radio telescopes, are the most extensive CO observations of this galaxy and are among the most extensive observations of molecular gas in any spiral galaxy. The molecular component in NGC6946 is unusually massive, with a ratio of molecular to atomic Hydrogen of 0.57. A star formation efficiency image for NGC6946 ranges by over two orders of magnitude with highest values found in the northeastern spiral arm, and anticorrelates with the 6cm polarized emission image, which traces the regular part of the magnetic field. We analyse the ISM in NGC6946's disk by making 1-D and 2-D comparisons of images made in several wavebands. A point-by-point correlation technique finds that the molecular gas is closely associated with the 7micron-emitting dust. The high correlation found between the MIR emission and the radio continuum at 6cm cannot be due to dust heating and gas ionization in star-forming regions because the thermal radio emission is less correlated with the MIR than the nonthermal emission. A coupling of magnetic fields to gas clouds is proposed as a possible scenario.

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arXiv2002

Red Giant Branch stars: the theoretical framework

Theoretical predictions of Red Giant Branch stars' effective temperatures, colors, luminosities and surface chemical abundances are a necessary tool for the astrophysical interpretation of the visible--near infrared integrated light from unresolved stellar populations, the Color-Magnitude-Diagrams of resolved stellar clusters and galaxies, and spectroscopic determinations of red giant chemical abundances. On the other hand, the comparison with empirical constraints provides a stringent test for the accuracy of present generations of red giant models. We review the current status of red giant stars' modelling, discussing in detail the still existing uncertainties affecting the model input physics (e.g., electron conduction opacity, treatment of the superadiabatic convection), and the adequacy of the physical assumptions built into the model computations. We compare theory with several observational features of the Red Giant Branch in galactic globular clusters, such as the luminosity function bump, the luminosity of the Red Giant Branch tip and the envelope chemical abundance patterns, to show the level of agreement between current stellar models and empirical data concerning the stellar luminosities, star counts, and surface chemical abundances.

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arXiv2001

The Interacting Dwarf Galaxy NGC 3077: The Interplay of Atomic and Molecular Gas with Violent Star Formation

We present a comprehensive multi-wavelength study of the nearby interacting dwarf galaxy NGC3077 (member of the M81 triplet). High resolution VLA HI observations show that most of the atomic gas (~90%) around NGC3077 is situated in a prominent tidal arm with a complex velocity structure. Little HI (~5 x 10^7 M_sun) is associated with NGC3077 itself. High resolution OVRO observations of the molecular component (CO) reveal the presence of 16 molecular complexes near the center of NGC3077 (total mass: ~1.6 x 10^6 M_sun). A virial mass analysis of the individual complexes yields a lower CO-to-H_2 conversion factor in NGC3077 than the Galactic value - a surprising result for a dwarf galaxy. The total (atomic and molecular) gas content in the centre of NGC3077 is displaced from the stellar component of NGC3077 -- this implies that not only the gas at large galactocentric radii is affected by the interaction within the triplet but also the center. We speculate that the starburst activity of NGC3077 was triggered by this redistribution of gas in the center. Some of these supershells are surrounded by neutral hydrogen. In a few cases, the rims of the ionized supershells are associated with dust absorption. The most prominent star forming region in NGC3077 as probed by Pa-alpha observations is hidden behind a dust cloud which is traced by the molecular complexes. Correcting for extinction we derive a star forming rate of 0.05 M_sun year^-1, i.e. given the reservoir in atomic and molecular gas in NGC3077, star formation may proceed at a similar rate for a few 10^8 years. The efficiency to form stars out of molecular gas in NGC3077 is similar to that in M82.

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arXiv2001

Evolution of Planetary Nebulae I. An improved synthetic model

We present a new synthetic model to follow the evolution of a planetary nebula (PN) and its central star, starting from the onset of AGB phase up to the white dwarf cooling sequence. The model suitably combines various analytical prescriptions to account for different (but inter-related) aspects of planetary nebulae, such as: the dynamical evolution of the primary shell and surrounding ejecta, the photoionisation of H and He by the central star, the nebular emission of a few relevant optical lines (e.g. Hbeta; HeII4686; [OIII]5007). Predictions of the synthetic model are tested by comparison with both findings of hydrodynamical calculations, and observations of Galactic PNe. The sensitiveness of the results to the models parameters (e.g. transition time, mass of the central star, H-/He-burning tracks, etc.) is also discussed. We briefly illustrate the systematic differences that are expected in the luminosities and lifetimes of PNe with either H- or He-burning central stars, which result in different ``detection probabilities'' across the H-R diagram, in both Hbeta and [OIII]5007 lines. Adopting reasonable values of the model parameters, we are able to reproduce, in a satisfactory way, many general properties of PNe, like the ionised mass--nebular radius relationship, the trends of a few main nebular line ratios, and the observed ranges of nebular shell thicknesses, electron densities, and expansion velocities. The models naturally predict also the possible transitions from optically-thick to optically-thin configurations (and vice versa). In this context, the origin of the Zanstra discrepancy is also analysed. (abridged)

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arXiv2001

First detection of ammonia in M82

We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion lines of ammonia (NH3) towards the south--western molecular lobe in M82. The relative intensities of the ammonia lines are characterized by a rotational temperature of T_rot=29+/-5 K which implies an average kinetic temperature of T_kin~60 K. A Gaussian decomposition of the observed spectra indicates increasing kinetic temperatures towards the nucleus of M82, consistent with recent findings based on CO observations. The observations imply a very low NH3 abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a decreasing NH3 abundance towards the central active regions in M82 and interpret this abundance gradient in terms of photodissociation of NH3 in PDRs. The low temperature derived here from NH3 also explains the apparent underabundance of complex molecules like CH_3OH and HNCO, which has previously been reported.

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arXiv2001

On the helium flash in low-mass Population III Red Giant stars

We investigate the evolution of initially metal-free, low-mass Red Giant stars through the He core flash at the tip of the Red Giant Branch. The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium flash driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering a H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon and nitrogen. Extending previous results by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of `real' stars, and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose Red Giant Branch progenitor has experienced the H-flash and canonical models. In particular, the Asymptotic Giant Branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.

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arXiv2001

Solar models and electron screening

We investigate the sensitivity of the solar model to changes in the nuclear reaction screening factors. We show that the sound speed profile as determined by helioseismology certainly rules out changes in the screening factors exceeding more than 10%. A slightly improved solar model could be obtained by enhancing screening by about 5% over the Salpeter value. We also discuss how envelope properties of the Sun depend on screening, too. We conclude that the solar model can be used to help settling the on-going dispute about the ``correct'' screening factors.

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arXiv2000

The effect of violent star formation on the state of the molecular gas in M82

We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M82. The analysis is based on the two lowest transitions of 12CO and the ground transition of the rare isotopes 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared peaks which trace emission from hot dust. The hot gas is associated with low H2 densities while the cold gas in the outer molecular lobes has high H2 densities. We find that CO intensities do not trace H2 column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H2 column density shows that X_CO depends on the excitation conditions. We find X_CO ~ Sqrt(n_H2)/T_kin, as expected for virialized clouds.

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