Chemical evolution of the Small Magellanic Cloud based on planetary nebulae

We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint of chemical evolution models of the SMC. We have used high quality PNe data in order to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula, and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used together with data available in the literature to improve the accuracy of the fluxes for each spectral line. We obtained accurate chemical abundances for PNe in the Small Magellanic Cloud, which can be useful as tools in the study of the chemical evolution of this galaxy and of Local Group galaxies. We present the resulting oxygen versus age diagram and a similar relation involving the [Fe/H] metallicity based on a correlation with stellar data. We discuss the implications of the derived age-metallicity relation for the SMC formation, in particular by suggesting a star formation burst in the last 2-3 Gyr.Comment: 11 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

The evolution of planetary nebulae IV. On the physics of the luminosity function

The nebular evolution is followed from the vicinity of the asymptotic-giant branch across the Hertzsprung-Russell diagram until the white-dwarf domain is reached, using various central-star models coupled to different initial envelope configurations. Along each sequence the relevant line emissions of the nebulae are computed and analysed. Maximum line luminosities in Hbeta and [OIII] 5007A are achieved at stellar effective temperatures of about 65000K and 95000-100000K, respectively, provided the nebula remains optically thick for ionising photons. In the optically thin case, the maximum line emission occurs at or shortly after the thick/thin transition. Our models suggest that most planetary nebulae with hotter (>~ 45000K) central stars are optically thin in the Lyman continuum, and that their [OIII] 5007A emission fails to explain the bright end of the observed planetary nebulae luminosity function. However, sequences with central stars of >~ 0.6 Msun and rather dense initial envelopes remain virtually optically thick and are able to populate the bright end of the luminosity function. Individual luminosity functions depend strongly on the central-star mass and on the variation of the nebular optical depth with time. Hydrodynamical simulations of planetary nebulae are essential for any understanding of the basic physics behind their observed luminosity function. In particular, our models do not support the claim of Marigo et.al (2004) according to which the maximum 5007A luminosity occurs during the recombination phase well beyond 100 000K when the stellar luminosity declines and the nebular models become, at least partially, optically thick. Consequently, there is no need to invoke relatively massive central stars of, say > 0.7 Msun, to account for the bright end of the luminosity function.Comment: 19 pages, 20 figures, A&A, in pres

Discovery of a [WO] central star in the planetary nebula Th 2-A

% context About 2500 planetary nebulae are known in our Galaxy but only 224 have central stars with reported spectral types in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Acker et al. 1992; Acker et al. 1996) % aims We have started an observational program aiming to increase the number of PN central stars with spectral classification. % methods By means of spectroscopy and high resolution imaging, we identify the position and true nature of the central star. We carried out low resolution spectroscopic observations at CASLEO telescope, complemented with medium resolution spectroscopy performed at Gemini South and Magellan telescopes. % results As a first outcome of this survey, we present for the first time the spectra of the central star of the PN Th 2-A. These spectra show emission lines of ionized C and O, typical in Wolf-Rayet stars. % conclusions We identify the position of that central star, which is not the brightest one of the visual central pair. We classify it as of type [WO 3]pec, which is consistent with the high excitation and dynamical age of the nebula.Comment: 3 pages and 2 figures. Paper recommended for publication in A&

AAOmega radial velocities rule out current membership of the planetary nebula NGC 2438 in the open cluster M46

We present new radial velocity measurements of 586 stars in a one-degree field centered on the open cluster M46, and the planetary nebula NGC 2438 located within a nuclear radius of the cluster. The data are based on medium-resolution optical and near-infrared spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope. We find a velocity difference of about 30 km/s between the cluster and the nebula, thus removing all ambiguities about the cluster membership of the planetary nebula caused by contradicting results in the literature. The line-of-sight velocity dispersion of the cluster is 3.9+/-0.3 km/s, likely to be affected by a significant population of binary stars.Comment: 6 pages + 5 figures, accepted for publication in MNRA

On the Origin of the Kinematical Differences Between the Stellar Halo and the Old Globular Cluster System in the Large Magellanic Cloud

We discuss structural and kinematical properties of the stellar halo and the old globular cluster system (GCS) in the Large Magellanic Cloud (LMC) based on numerical simulations of the LMC formation. We particularly discuss the observed possible GCS's rotational kinematics (V/sigma > 2) that appears to be significantly different from the stellar halo's one with a large velocity dispersion (~50 km/s). We consider that both halo field stars and old GCs can originate from low-mass subhalos virialized at high redshifts (z >6). We investigate the final dynamical properties of the two old components in the LMC's halo formed from merging of low-mass subhalos with field stars and GCs. We find that the GCS composed of old globular clusters (GCs) formed at high redshifts (z > 6) has little rotation (V/sigma ~0.4) and structure and kinematics similar to those of the stellar halo. This inconsistency between the simulated GCS's kinematics and the observed one is found to be seen in models with different parameters. This inconsistency therefore implies that if old, metal-poor GCs in the LMC have rotational kinematics, they are highly unlikely to originate from the low-mass subhalos that formed the stellar halo. We thus discuss a scenario in which the stellar halo was formed from low-mass subhalos with no/few GCs whereas the GCS was formed at the very early epoch of the LMC's disk formation via dissipative minor and major merging of gas-rich subhalos and gas infall. We suggest that there can be a threshold subhalo mass above which GCs can be formed within subhalos at high redshifts and thus that this threshold causes differences in physical properties between stellar halos and GCSs in less luminous galaxies like the LMC.Comment: 10 pages, 8 figures, accepted by MNRA

The Distance Scale of Planetary Nebulae

By collecting distances from the literature, a set of 73 planetary nebulae with mean distances of high accuracy is derived. This sample is used for recalibration of the mass-radius relationship, used by many statistical distance methods. An attempt to correct for a statistical peculiarity, where errors in the distances influences the mass--radius relationship by increasing its slope, has been made for the first time. Distances to PNe in the Galactic Bulge, derived by this new method as well as other statistical methods from the last decade, are then used for the evaluation of these methods as distance indicators. In order of achieving a Bulge sample that is free from outliers we derive new criteria for Bulge membership. These criteria are much more stringent than those used hitherto, in the sense that they also discriminate against background objects. By splitting our Bulge sample in two, one with optically thick (small) PNe and one with optically thin (large) PNe, we find that our calibration is of higher accuracy than most other calibrations. Differences between the two subsamples, we believe, are due to the incompleteness of the Bulge sample, as well as the dominance of optical diameters in the ``thin'' sample and radio diameters in the ``thick'' sample. Our final conclusion is that statistical methods give distances that are at least as accurate as the ones obtained from many individual methods. Also, the `long' distance scale of Galactic PNe is confirmed.Comment: 15 pages, 9 figures, accepted for publication in A&

The star cluster formation history of the LMC

The Large Magellanic Cloud is one of the nearest galaxies to us and is one of only few galaxies where the star formation history can be determined from studying resolved stellar populations. We have compiled a new catalogue of ages, luminosities and masses of LMC star clusters and used it to determine the age distribution and dissolution rate of LMC star clusters. We find that the frequency of massive clusters with masses M>5000 Msun is almost constant between 10 and 200 Myr, showing that the influence of residual gas expulsion is limited to the first 10 Myr of cluster evolution or clusters less massive than 5000 Msun. Comparing the cluster frequency in that interval with the absolute star formation rate, we find that about 15% of all stars in the LMC were formed in long-lived star clusters that survive for more than 10 Myr. We also find that the mass function of LMC clusters younger than 1 Gyr can be fitted by a power-law mass function with slope \alpha=-2.3, while older clusters follow a significantly shallower slope and interpret this is a sign of the ongoing dissolution of low-mass clusters. Our data shows that for ages older than 200 Myr, about 90% of all clusters are lost per dex of lifetime. The implied cluster dissolution rate is significantly faster than that based on analytic estimates and N-body simulations. Our cluster age data finally shows evidence for a burst in cluster formation about 1 Gyr ago, but little evidence for bursts at other ages.Comment: 18 pages, 6 figures, MNRAS in pres

Gamma-ray and synchrotron emission from neutralino annihilation in the Large Magellanic Cloud

We calculate the expected flux of gamma-ray and radio emission from the LMC due to neutralino annihilation. Using rotation curve data to probe the density profile and assuming a minimum disk, we describe the dark matter halo of the LMC using models predicted by N-body simulations. We consider a range of density profiles including the NFW profile, a modified NFW profile proposed by Hayashi et al.(2003) to account for the effects of tidal stripping, and an isothermal sphere with a core. We find that the gamma-ray flux expected from these models may be detectable by GLAST for a significant part of the neutralino parameter space. The prospects for existing and upcoming Atmospheric Cherenkov Telescopes are less optimistic, as unrealistically long exposures are required for detection. However, the effects of adiabatic compression due to the baryonic component may improve the chances for detection by ACTs. The maximum flux we predict is well below EGRET's measurements and thus EGRET does not constrain the parameter space. The expected synchrotron emission generally lies below the observed radio emission from the LMC in the frequency range of 19.7 to 8550 MHz. As long as <2x 10^-26 cm^3 s^-1 for a neutralino mass of 50 GeV, the observed radio emission is not primarily due to neutralinos and is consistent with the assumption that the main source is cosmic rays. We find that the predicted fluxes, obtained by integrating over the entire LMC, are not very strongly dependent on the inner slope of the halo profile, varying by less than an order of magnitude for the range of profiles we considered.Comment: 24 pages, 5 figures; detailed discussion of how the neutralino induced signals compare with the cosmic-ray induced ones was added. Main conclusions unchanged. Matches accepted version, to appear in Astroparticle Physic

A kinematic study of planetary nebulae in the dwarf irregular galaxy IC10

We present positions, kinematics, and the planetary nebula luminosity function (PNLF) for 35 planetary nebulae (PNe) in the nearest starburst galaxy IC10 extending out to 3kpc from the galaxy's centre. We take advantage of the deep imaging and spectroscopic capabilities provided by the spectrograph FOCAS on the 8.2m Subaru telescope. The PN velocities were measured through the slitless-spectroscopy technique, which allows us to explore the kinematics of IC10 with high precision. Using these velocities, we conclude that there is a kinematic connection between the HI envelope located around IC10 and the galaxy's PN population. By assuming that the PNe in the central regions and in the outskirts have similar ages, our results put strong observational constraints on the past tidal interactions in the Local Group. This is so because by dating the PN central stars, we, therefore, infer the epoch of a major episode of star formation likely linked to the first encounter of the HI extended envelope with the galaxy. Our deep [OIII] images also allow us to use the PNLF to estimate a distance modulus of 24.1+/-0.25, which is in agreement with recent results in the literature based on other techniques.Comment: 10 pages, 9 figures, 2 tables. Accepted for publication in MNRA

A New Population of Planetary Nebulae Discovered in the Large Magellanic Cloud (III): The Luminosity Function

Our previous identification and spectroscopic confirmation of 431 faint, new planetary nebulae in the central 25 deg^2 region of the LMC permits us to now examine the shape of the LMC Planetary Nebula Luminosity Function (PNLF) through an unprecedented 10 magnitude range. The majority of our newly discovered and previously known PNe were observed using the 2dF, multi-object fibre spectroscopy system on the 3.9-m Anglo-Australian Telescope and the FLAMES multi-object spectrograph on the 8-m VLT. We present reliable [OIII]5007 and H-beta flux estimates based on calibrations to well established PN fluxes from previous surveys and spectroscopic standard stars. The bright cutoff (M*) of the PNLF is found by fitting a truncated exponential curve to the bright end of the PNLF over a 3.4 magnitude range. This cutoff is used to estimate a new distance modulus of 18.46 to the LMC, in close agreement with previous PNLF studies and the best estimates by other indicators. The bright end cutoff is robust to small samples of bright PNe since significantly increased PN samples do not change this fiducial. We then fit a truncated exponential curve directly to the bright end of the function over a 6 magnitude range and test the curve's ability to indicate the position of M*. Because of the significant increase in the number of LMC PN, the shape of the PNLF is now examined in greater detail than has previously been possible. Through cumulative functions, the new LMC PNLF is compared to those from the SMC and a new deep local Galactic sample revealing the effects of incompleteness. The new [OIII]5007 LMC PNLF is then compared to our new H-beta LMC PNLF using calibrated and measured fluxes for the same objects, revealing the effects of metallicity on the [OIII]5007 line.Comment: 28 pages, 24 figures. Accepted MNRA