The type IIb SN 2008ax: the nature of the progenitor

A source coincident with the position of the type IIb supernova (SN) 2008ax is identified in pre-explosion Hubble Space Telescope (HST) Wide Field Planetary Camera 2 observations in three optical filters. We identify and constrain two possible progenitor systems: (i) a single massive star that lost most of its hydrogen envelope through radiatively driven mass-loss processes, prior to exploding as a helium-rich Wolf-Rayet star with a residual hydrogen envelope, and (ii) an interacting binary in a low-mass cluster producing a stripped progenitor. Late time, high-resolution observations along with detailed modelling of the SN will be required to reveal the true nature of this progenitor star.

Core-collapse supernovae in low-metallicity environments and future all-sky transient surveys

Aims. Massive stars in low-metallicity environments may produce exotic explosions such as long-duration gamma-ray bursts and pair-instability supernovae when they die as core-collapse supernovae (CCSNe). Such events are predicted to be relatively common in the early Universe during the first episodes of star-formation. To understand these distant explosions it is vital to study nearby CCSNe arising in low-metallicity environments to determine if the explosions have different characteristics to those studied locally in high-metallicity galaxies. Many of the nearby supernova searches concentrate their efforts on high star-formation rate galaxies, hence biasing the discoveries to metal rich regimes. Here we determine the feasibility of searching for these CCSNe in metal-poor dwarf galaxies using various survey strategies.

The metamorphosis of supernova SN 2008D/XRF 080109: A link between supernovae and GRBs/hypernovae

The only supernovae (SNe) to show gamma-ray bursts ( GRBs) or early x-ray emission thus far are overenergetic, broad- lined type Ic SNe ( hypernovae, HNe). Recently, SN 2008D has shown several unusual features: (i) weak x-ray flash (XRF), (ii) an early, narrow optical peak, (iii) disappearance of the broad lines typical of SN Ic HNe, and (iv) development of helium lines as in SNe Ib. Detailed analysis shows that SN 2008D was not a normal supernova: Its explosion energy (E approximate to 6 x 10(51) erg) and ejected mass [similar to 7 times the mass of the Sun ( M.)] are intermediate between normal SNe Ibc and HNe. We conclude that SN 2008D was originally a similar to 30 M. star. When it collapsed, a black hole formed and a weak, mildly relativistic jet was produced, which caused the XRF. SN 2008D is probably among the weakest explosions that produce relativistic jets. Inner engine activity appears to be present whenever massive stars collapse to black holes.

A deeper search for the progenitor of the Type Ic supernova 2002ap

Images of the site of the Type Ic supernova (SN) 2002ap taken before explosion were analysed previously by Smartt et al. We have uncovered new unpublished, archival pre-explosion images from the Canada-France-Hawaii Telescope (CFHT) that are vastly superior in depth and image quality. In this paper we present a further search for the progenitor star of this unusual Type Ic SN. Aligning high-resolution Hubble Space Telescope observations of the SN itself with the archival CFHT images allowed us to pinpoint the location of the progenitor site on the groundbased observations. We find that a source visible in the B- and R-band pre-explosion images close to the position of the SN is (1) not coincident with the SN position within the uncertainties of our relative astrometry and (2) is still visible similar to 4.7-yr post-explosion in late-time observations taken with the William Herschel Telescope. We therefore conclude that it is not the progenitor of SN 2002ap. We derived absolute limiting magnitudes for the progenitor of M-B >= -4.2 +/- 0.5 and M-R >= -5.1 +/- 0.5. These are the deepest limits yet placed on a Type Ic SN progenitor. We rule out all massive stars with initial masses greater than 7-8 M-circle dot (the lower mass limit for stars to undergo core collapse) that have not evolved to become Wolf-Rayet stars. This is consistent with the prediction that Type Ic SNe should result from the explosions of Wolf-Rayet stars. Comparing our luminosity limits with stellar models of single stars at appropriate metallicity (Z = 0.008) and with standard mass-loss rates, we find no model that produces a Wolf-Rayet star of low enough mass and luminosity to be classed as a viable progenitor. Models with twice the standard mass-loss rates provide possible single star progenitors but all are initially more massive than 30-40 M-circle dot. We conclude that any single star progenitor must have experienced at least twice the standard mass-loss rates, been initially more massive than 30-40 M-circle dot and exploded as a Wolf-Rayet star of final mass 10-12 M-circle dot. Alternatively a progenitor star of lower initial mass may have evolved in an interacting binary system. Mazzali et al. propose such a binary scenario for the progenitor of SN 2002ap in which a star of initial mass 15-20 M-circle dot is stripped by its binary companion, becoming a 5 M-circle dot Wolf-Rayet star prior to explosion. We constrain any possible binary companion to a main-sequence star of

The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

We present a comprehensive study of the observational dependence of the mass-loss rate in stationary stellar winds of hot massive stars on the metal content of their atmospheres. The metal content of stars in the Magellanic Clouds is discussed, and a critical assessment is given of state-of-the-art mass-loss determinations of OB stars in these two satellite systems and the Milky-Way. Assuming a power-law dependence of mass loss on metal content,. M. Z(m), and adopting a theoretical relation between the terminal flow velocity and metal content, v(infinity). Z(0.13) (Leitherer et al. 1992, ApJ, 401, 596), we find m = 0.83 +/- 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 105.2 L circle dot. Within the errors, this result is in agreement with the prediction m = 0.69 +/- 0.10 by Vink et al. (2001, A& A, 369, 574). Absolute empirical values for the mass loss, based on Ha and ultraviolet (UV) wind lines, are found to be a factor of two higher than predictions in this high luminosity regime. If this difference is attributed to inhomogeneities in the wind, and this clumping does not impact the predictions, this would imply that luminous O and early-B stars have clumping factors in their Ha and UV line forming regions of about a factor of four. For lower luminosity stars, the winds are so weak that their strengths can generally no longer be derived from optical spectral lines (essentially Ha) and one must currently rely on the analysis of UV lines. We confirm that in this low-luminosity domain the observed Galactic WLR is found to be much steeper than expected from theory (although the specific sample is rather small), leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L similar to 10(4.75) L circle dot, the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars would turn out to be overestimated as a result of wind clumping, but the degree of clumping would be rather independent of metallicity, the scalings derived in this study are expected to remain correct.

Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gm

Photometric and spectroscopic observations of the faint Supernovae (SNe) 2002kg and 2003gm, and their precursors, in NGC 2403 and NGC 5334, respectively, are presented. The properties of these SNe are discussed in the context of previously proposed scenarios for faint SNe: low-mass progenitors producing underenergetic SNe; SNe with ejecta constrained by a circumstellar medium; and outbursts of massive Luminous Blue Variables (LBVs). The last scenario has been referred to as 'Type V SNe', 'SN impostors' or 'fake SNe'.

Blue luminous stars in nearby galaxies: Quantitative spectral analysis of M33 B-type supergiant stars

We present the detailed spectral analysis of a sample of M33 B-type supergiant stars, aimed at the determination of their fundamental parameters and chemical composition. The analysis is based on a grid of non-LTE metal line-blanketed model atmospheres including the effects of stellar winds and spherical extension computed with the code FASTWIND. Surface abundance ratios of C, N, and O are used to discuss the chemical evolutionary status of each individual star. The comparison of observed stellar properties with theoretical predictions of massive star evolutionary models shows good agreement within the uncertainties of the analysis. The spatial distribution of the sample allows us to investigate the existence of radial abundance gradients in the disk of M33. The comparison of stellar and H II region O abundances ( based on direct determinations of the electron temperature of the nebulae) shows good agreement. Using a simple linear radial representation, the stellar oxygen abundances result in a gradient of -0.0145 +/- 0.005 dex arcmin(-1) (or -0.06 +/- 0.02 dex kpc(-1)) up to a distance equal to similar to 1.1 times the isophotal radius of the galaxy. A more complex representation cannot be completely discarded by our stellar sample. The stellar Mg and Si abundances follow the trend displayed by O abundances, although with shallower gradients. These differences in gradient slope cannot be explained at this point. The derived abundances of the three alpha-elements yield solar metallicity in the central regions of the disk of M33. A comparison with recent planetary nebula data from Magrini and coworkers indicates that the disk of M33 has not suffered from a significant O enrichment in the last 3 Gyr.

POINT-AGAPE pixel lensing survey of M31 - Evidence for a MACHO contribution to galactic halos

The POINT-AGAPE collaboration is carrying out a search for gravitational microlensing toward M31 to reveal galactic dark matter in the form of MACHOs (Massive Astrophysical Compact Halo Objects) in the halos of the Milky Way and M31. A high-threshold analysis of 3 years of data yields 6 bright, short- duration microlensing events, which are confronted to a simulation of the observations and the analysis. The observed signal is much larger than expected from self lensing alone and we conclude, at the 95% confidence level, that at least 20% of the halo mass in the direction of M31 must be in the form of MACHOs if their average mass lies in the range 0.5-1 M-circle dot. This lower bound drops to 8% for MACHOs with masses similar to 0.01 M-circle dot. In addition, we discuss a likely binary microlensing candidate with caustic crossing. Its location, some 32' away from the centre of M31, supports our conclusion that we are detecting a MACHO signal in the direction of M31.

Theory of pixel lensing toward M31. II. The velocity anisotropy and flattening of the MACHO distribution

The POINT-AGAPE collaboration is currently searching for massive compact halo objects (MACHOs) toward the Andromeda galaxy (M31). The survey aims to exploit the high inclination of the M31 disk, which causes an asymmetry in the spatial distribution of M31 MACHOs. Here, we investigate the effects of halo velocity anisotropy and flattening on the asymmetry signal using simple halo models. For a spherically symmetric and isotropic halo, we find that the underlying pixel lensing rate in far-disk M31 MACHOs is more than 5 times the rate of near-disk events. We find that the asymmetry is further increased by about 30% if the MACHOs occupy radial orbits rather than tangential orbits, but it is substantially reduced if the MACHOs lie in a flattened halo. However, even for halos with a minor- to major-axis ratio of q = 0.3, the number of M31 MACHOs in the far side outnumber those in the near side by a factor of similar to2. There is also a distance asymmetry, in that the events on the far side are typically farther from the major axis. We show that, if this positional information is exploited in addition to number counts, then the number of candidate events required to confirm asymmetry for a range of flattened and anisotropic halo models is achievable, even with significant contamination by variable stars and foreground microlensing events. For pixel lensing surveys that probe a representative portion of the M31 disk, a sample of around 50 candidates is likely to be sufficient to detect asymmetry within spherical halos, even if half the sample is contaminated, or to detect asymmetry in halos as flat as q = 0.3, provided less than a third of the sample comprises contaminants. We also argue that, provided its mass-to-light ratio is less than 100, the recently observed stellar stream around M31 is not problematic for the detection of asymmetry.

The chemical composition of two supergiants in the dwarf irregular galaxy WLM

The chemical composition of two stars in WLM has been determined from high-quality Ultraviolet-Visual Echelle Spectrograph (UVES) data obtained at the VLT-UT2. The model atmospheres analysis shows that they have the same metallicity, [Fe/H] = - 0.38 +/- 0.20 (+/- 0.29). Reliable magnesium abundances are determined from several lines of two ionization states in both stars resulting in [Mg/Fe] = - 0.24 +/- 0.16 (+/- 0.28). This result suggests that the [alpha(Mg)/Fe] ratio in WLM may be suppressed relative to solar abundances ( also supported by differential abundances relative to similar stars in NGC 6822 and the Small Magellanic Cloud [SMC]). The absolute Mg abundance, [Mg/H] = -0.62, is high relative to what is expected from the nebulae though, where two independent spectroscopic analyses of the H II regions in WLM yield [O/H] = - 0.89. Intriguingly, the oxygen abundance determined from the O I lambda6158 feature in one WLM star is [O/H] = - 0.21 +/- 0.10 (+/- 0.05), corresponding to 5 times higher than the nebular oxygen abundance. This is the first time that a significant difference between stellar and nebular oxygen abundances has been found, and currently, there is no simple explanation for this difference. The two stars are massive supergiants with distances that clearly place them in WLM. They are young ( less than or equal to 10 Myr) and should have a similar composition to the ISM. Additionally, differential abundances suggest that the O/Fe ratio in the WLM star is consistent with similar stars in NGC 6822 and the SMC, galaxies where the average stellar oxygen abundances are in excellent agreement with the nebular results. If the stellar abundances reflect the true composition of WLM, then this galaxy lies well above the metallicity-luminosity relationship for dwarf irregular galaxies. It also suggests that WLM is more chemically evolved than currently interpreted from its color-magnitude diagram. The similarities between the stars in WLM and NGC 6822 suggest that these two galaxies may have had similar star formation histories.

Mass limits for the progenitor star of supernova 2001du and other Type II-P supernovae

The supernova SN 2001du was discovered in the galaxy NGC 1365 at a distance of 19 +/- 2 Mpc, and is a core-collapse event of Type II-P. Images of this galaxy, of moderate depth, have been taken with the Hubble Space Telescope approximately 6.6 yr before discovery and include the supernova position on the WFPC2 field of view. We have observed the supernova with the WFPC2 to allow accurate differential astrometry of SN 2001du on the pre-explosion frames. As a core-collapse event it is expected that the progenitor was a massive, luminous star. There is a marginal detection (3sigma) of a source close to the supernova position on the pre-discovery V -band frame, but it is not precisely coincident and we do not believe it to be a robust detection of a point source. We conclude that there is no stellar progenitor at the supernova position and derive sensitivity limits of the pre-discovery images that provide an upper mass limit for the progenitor star. We estimate that the progenitor had a mass of less than 15 M-circle dot . We revisit two other nearby Type II-P supernovae that have high-quality pre-explosion images, and refine the upper mass limits for the progenitor stars. Using a new distance determination for SN 1999gi from the expanding photosphere method, we revise the upper mass limit to 12 M-circle dot . We present new HST images of the site of SN 1999em, which validate the use of lower spatial resolution ground-based images in the progenitor studies and use a new Cepheid distance to the galaxy to measure an upper mass limit of 15 M-circle dot for that progenitor. Finally we compile all the direct information available for the progenitors of eight nearby core-collapse supernovae and compare their mass estimates. These are compared with the latest stellar evolutionary models of pre-supernova evolution which have attempted to relate metallicity and mass to the supernovae type. Although this is statistically limited at present, reasonable agreement is already found for the lower-mass events (generally the II-P), but some discrepancies appear at higher masses.

An upper mass limit for the progenitor of the Type II-P supernova SN 1999gi

Masses and progenitor evolutionary states of Type II supernovae remain almost unconstrained by direct observations. Only one robust observation of a progenitor (SN 1987A) and one plausible observation (SN 1993J) are available. Neither matched theoretical predictions, and in this Letter we report limits on a third progenitor (SN 1999gi). The Hubble Space Telescope (HST) has imaged the site of the Type II-P supernova SN 1999gi with the Wide Field Planetary Camera 2 (WFPC2) in two filters (F606W and F300W) prior to explosion. The distance to the host galaxy (NGC 3184) of 7.9 Mpc means that the most luminous, massive stars are resolved as single objects in the archive images. The supernova occurred in a resolved, young OB association 2.3 kpc from the center of NGC 3184 with an association age of about 4 Myr. Follow-up images of SN 1999gi with WFPC2 taken 14 months after discovery determine the precise position of the supernova on the preexplosion frames. An upper limit of the absolute magnitude of the progenitor is estimated (M-v greater than or equal to -5.1). By comparison with stellar evolutionary tracks, this can be interpreted as a stellar mass, and we determine an upper mass limit of 9(-2)(+3) M.. We discuss the possibility of determining the masses or mass limits for numerous nearby core-collapse supernovae using the HST archive enhanced by our current SNAP program.

A short-timescale candidate microlensing event in the point-agape pixel lensing survey of M31

We report the discovery of a short-duration microlensing candidate in the northern field of the POINT-AGAPE pixel lensing survey toward M31. Almost certainly, the source star has been identified on Hubble Space Telescope archival images, allowing us to infer an Einstein crossing time of t(E) = 10.4 days, a maximum magnification of A(max) similar to 18, and a lens-source proper motion mu (rel) > 0.3 mu as day(-1). The event has a projected separation of 8' from the center of M31, beyond the bulk of the stellar lens population. There are three plausible identifications/locations for the lensing object: a massive compact halo object (MACHO) in either M31 or the Milky Way, or a star in the M31 disk. The most probable mass is 0.06 M-. for an M31 MACHO, 0.02 M-. for a Milky Way MACHO, and 0.2 M-. for an M31 stellar lens. While the stellar interpretation is possible, the MACHO interpretation is the most probable for halo fractions above 20%.

Theory of pixel lensing towards M31 - I. The density contribution and mass of MACHOs

POINT-AGAPE is an Angle-French collaboration which is employing the Isaac Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. Pixel lensing is a technique which permits the detection of microlensing against unresolved stellar fields. The survey aims to constrain the stellar population in M31, and also the distribution and nature of massive compact halo objects (MACHOs) in both M31 and the Galaxy.

The WR content of IC10 - first detection of WC9 stars in a low metallicity environment?

We present deep, narrow-hand photometry of the Local Group starburst galaxy IC10. Our dedicated photometric system provides detection of 13 new Wolf-Rayet (WR) stars and allows spectral subtypes to be assigned. Three of these new stars appear to be WC9 subtypes. If confirmed, these would be the very first WC9 stars ever detected in a low metallicity environment, hence putting strong new constraints on the formation and evolution models of massive stars. Eight of the new WR stars are of the WC subtype, which does not significantly modify the anomalously high WC/WN ratio in IC10. However it is likely that a number of Wolf-Rayet stars of the WNE and WC spectral subtypes are still to be discovered in the heart of the galaxy.