Robert J. Cumming

The Type IIn supernova 1994W: evidence for the explosive ejection of a circumstellar envelope

We present and analyse spectra of the Type IIn supernova (SN) 1994W obtained between 18 and 203d after explosion. During the luminous phase (first 100 d) the line profiles are composed of three major components: (i) narrow P-Cygni lines with the absorption minima at -700 km s -1; (ii) broad emission lines with blue velocity at zero intensity ∼4000km s -1; and (iii) broad, smooth wings extending out to at least ∼5000kms -1, most apparent in Hα. These components are identified with an expanding circumstellar (CS) envelope, shocked cool gas in the forward post-shock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P-Cygni lines from the SN is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the density (n ≈ 10 9cm -3), radial extent [∼(4-5) × 10 15cm] and Thomson optical depth (τ T ≳ 2.5) of the CS envelope during the first month. The plateau-like SN light curve is reproduced by a hydrodynamical model and is found to be powered by a combination of internal energy leakage after the explosion of an extended pre-SN (∼10 15 cm) and subsequent luminosity from CS interaction. The pre-explosion kinematics of the CS envelope is recovered, and is close to homologous expansion with outer velocity ∼1100 km s -1 and a kinematic age of ∼1.5 yr. The high mass (∼0.4M⊙) and kinetic energy (∼2 × 10 48 erg) of the CS envelope, combined with low age, strongly suggest that the CS envelope was explosively ejected ∼1.5 yr prior to the SN explosion.

A VERY LOW MASS OF 56Ni IN THE EJECTA OF SN 1994W

We present spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W. After the plateau phase days), the light curve dropped by D3.5 mag (t Z 120 in V in only 12 days. Between 125 and 197 days after explosion, the supernova faded substantially faster than the decay rate of 56Co, and by day 197 it was 3.6 mag less luminous in R than SN 1987A. The low R luminosity could indicate of 56Ni ejected at the explosion. The emission between (0.0026 ~0.00110.0017 M _ 125 and 197 days would in this case be dominated by di†usion of emission from the mantle region, or by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve could have been dominated by 56Co decay. In this case, the mass of the ejected 56Ni would have been and the rapid fading between 125 and 197 days most likely due to dust formation. 0.015 ~0.0080.012 M _ , Although this value of the mass is higher than in the -rst case, it is still lower than estimated for any previous Type II supernova. Only progenitors with andWe present spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W. After the plateau phase (t 120 days), the light curve dropped by ~3.5 mag in V in only 12 days. Between 125 and 197 days after explosion, the supernova faded substantially faster than the decay rate of 56Co, and by day 197 it was 3.6 mag less luminous in R than SN 1987A. The low R luminosity could indicate 0.0026+ 0.0017−0.0011 M☉ of 56Ni ejected at the explosion. The emission between 125 and 197 days would in this case be dominated by diffusion of emission from the mantle region, or by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve could have been dominated by 56Co decay. In this case, the mass of the ejected 56Ni would have been 0.015+ 0.012−0.008 M☉, and the rapid fading between 125 and 197 days most likely due to dust formation. Although this value of the mass is higher than in the first case, it is still lower than estimated for any previous Type II supernova. Only progenitors with MZAMS ~ 8-10 M☉ and MZAMS 25 M☉ are expected to eject such low masses of 56Ni. If MZAMS ~ 8-10 M☉, the plateau phase indicates a low explosion energy, while for a progenitor with MZAMS 25 M☉, the energy can be the canonical ~1051 ergs. As SN 1994W was unusually luminous, the low-mass explosion may require an uncomfortably high efficiency in converting explosion energy into radiation. This favors a MZAMS 25 M☉ progenitor. The supernovas narrow (~1000 km s-1) emission lines were excited by the hot supernova spectrum, rather than by a circumstellar shock. The thin shell from which the lines originated was most likely accelerated by radiation from the supernova.

Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess

We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases ( 8000A which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ~38 per cent of the galactic star formation rate.

SN 2009kn – the twin of the Type IIn supernova 1994W

We present an optical and near-infrared photometric and spectroscopic study of supernova (SN) 2009kn spanning ˜1.5 yr from the discovery. The optical spectra are dominated by the narrow (full width at half-maximum ˜1000 km s-1) Balmer lines distinctive of a Type IIn SN with P Cygni profiles. Contrarily, the photometric evolution resembles more that of a Type IIP SN with a large drop in luminosity at the end of the plateau phase. These characteristics are similar to those of SN 1994W, whose nature has been explained with two different models with different approaches. The well-sampled data set on SN 2009kn offers the possibility to test these models, in the case of both SN 2009kn and SN 1994W. We associate the narrow P Cygni lines with a swept-up shell composed of circumstellar matter and SN ejecta. The broad emission line wings, seen during the plateau phase, arise from internal electron scattering in this shell. The slope of the light curve after the post-plateau drop is fairly consistent with that expected from the radioactive decay of 56Co, suggesting an SN origin for SN 2009kn. Assuming radioactivity to be the main source powering the light curve of SN 2009kn in the tail phase, we infer an upper limit for 56Ni mass of 0.023 Msun. This is significantly higher than that estimated for SN 1994W, which also showed a much steeper decline of the light curve after the post-plateau drop. We also observe late-time near-infrared emission which most likely arises from newly formed dust produced by SN 2009kn. As with SN 1994W, no broad lines are observed in the spectra of SN 2009kn, not even in the late-time tail phase.

Circumstellar Hα from SN 1994D and future Type Ia supernovae: an observational test of progenitor models

Searching for the presence of circumstellar material is currently the only direct way to discriminate between the different types of possible progenitor systems for Type Ia supernovae. We have therefore looked for narrow Hin a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 days before maximum and only 6.5 days after explosion. We derive an upper limit of 2.0×10 −16 erg s −1 cm −2 for an unresolved emission line at the local H ii region velocity. To estimate the limit this puts on wind density, we have made time-dependent photoionization calculations. Assuming spherical symmetry we find an upper limit of u M � 1.5 × 10 −5 M⊙ yr −1 for a wind speed of 10 km s −1 . This limit can exclude only the highest-mass-loss-rate symbiotic systems as progenitors. We discuss the effect of asymmetry and assess the relative merits of early optical, radio and X-ray limits in constraining mass loss from Type Ia progenitors. We find that X-ray observations can probably provide the most useful limits on the progenitor mass loss, while high-resolution optical spectroscopy offers our only chance of actually identifying circumstellar hydrogen.

The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey - A ground-based follow-up survey of CO(1–0), CO(2–1), and CO(3–2)

Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims. We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNE Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [CII] 157 mu m and [OI] 63 mu m observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H-2 conversion factor X-CO (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results. We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z similar to 0.4 Z(circle dot)), Mrk 930 (0.2 Z(circle dot)), and UM 311 (0.5 Z(circle dot)), but CO remains undetected in NGC 4861 (0.2 Z(circle dot)). The CO luminosities are low, while [CII] is bright in these galaxies, resulting in [CII]/CO(1-0) >= 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled X-CO factors. Those galaxies are dominated by their HI gas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent X-CO factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies.

Dynamical masses of two young globular clusters in the blue compact galaxy ESO 338–IG04

We present high-resolution echelle spectroscopy, obtained with the UVES spectrograph on ESO/VLT, of two luminous star clusters in the metal-poor blue compact galaxy ESO 338-IG04 at a distance of 37 ...

Stellar dynamics of blue compact galaxies. II. Further indications of a merger in ESO 338-IG04

Luminous blue compact galaxies, common at z~1 but now relatively rare, show disturbed kinematics in emission lines. As part of a program to understand their formation and evolution, we have investigated the stellar dynamics of a number of nearby objects in this class. We have obtained long-slit spectra with VLT/FORS2 in the spectral region covering the near-infrared calcium triplet. In this paper we focus on the well known luminous blue compact galaxy ESO 338-IG04 (Tololo 1924-416). A previous investigation, using Fabry-Perot interferometry, showed that this galaxy has a chaotic H-alpha velocity field, indicating that either the galaxy is not in dynamical equilibrium, or that H-alpha does not trace the gravitational potential due to feedback from star formation. We show that along the apparent major axis, the stellar and ionised gas velocities follow each other closely. Hence, the chaotic velocity field must be attributed to the fact that the young stellar population in ESO 338-IG04 is not in dynamical equilibrium. The most likely explanation, which is also supported by its morphology, is that the galaxy has experienced a merger and that this has triggered the current starburst. We also reanalyse the rotation curve of ESO 400-G43, where the gas and stars are instead decoupled. Summarising the results of our program so far, we note that emission-line velocity fields are not always reliable tracers of stellar motions, and assess the implications for kinematic studies of similar galaxies at intermediate redshift.

Star-gas decoupling and a non-rotating stellar core in He 2-10: Integral field spectroscopy with FLAMES/ARGUS

We study the two-dimensional distribution and kinematics of the stellar and gaseous components in the centre of the blue compact dwarf galaxy, He 2-10. The aim is to compare the kinematics of gas and stars in order to determine whether they are consistent with one another, or if stars and gas can be decoupled due to gravitational perturbations and feedback from star formation. We have used the integral field unit ARGUS, part of FLAMES on the European Southern Observatorys Very Large Telescope, to target the near-infrared Ca-triplet in the central 300 times 480 parsecs of He 2-10. The selected wavelength regime includes several prominent spectral features, including the Paschen series and an [SIII] emission-line, which we have used to derive the kinematics of the ionised interstellar medium. We find no systematic trend in the velocities of the stars over the observed field of view and conclude that the stellar kinematics is governed by random motions. This is in contrast to the motions the ionised interstellar medium, where we find spatial velocity variations up to 60 km/s. Our gas velocity field is consistent with previous studies of both the molecular gas and the feedback-driven outflow in He 2-10. We interpret the kinematic decoupling between the stars and the gas as He 2-10 being in the process of transformation to a dwarf elliptical galaxy.

Stellar dynamics of blue compact galaxies - I. Decoupled star–gas kinematics in ESO 400-G43

In this letter we present the first results from a program aimed at measuring the stellar kinematics of blue compact galaxies by observing the near-infrared Calcium triplet. We show the first results for ESO 400-G43 based on deep VLT/FORS2 spectroscopy. The instabilities found in the central gaseous velocity field are not seen in the solid body stellar rotation curve, indicating that stars and gas are kinematically decoupled in this galaxy. Even if this galaxy has a perturbed gaseous velocity field, the stellar velocity dispersion on average agrees well with that derived from the nebular lines.