Interstellar absorptions towards the LMC: Small-scale density variations in Milky Way disc gas

Observations show that the ISM contains sub-structure on scales less than 1 pc, detected in the form of spatial and temporal variations in column densities or optical depth. Despite the number of detections, the nature and ubiquity of the small-scale structure in the ISM is not yet fully understood. We use UV absorption data mainly from FUSE and partly from STIS of six LMC stars (Sk-67{\deg}111, LH54-425, Sk-67{\deg}107, Sk-67{\deg}106, Sk-67{\deg}104, and Sk-67{\deg}101), all located within 5 arcmin of each other, and analyse the physical properties of the Galactic disc gas in front of the LMC on sub-pc scales. We analyse absorption lines of a number of ions within the UV spectral range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and fine-structure levels of neutral carbon have been used in order to study changes in the density and the physical properties of the Galactic disc gas over small angular scales. While most species do not show any significant variation in their column densities, we find an enhancement of almost 2 dex for H_2 from Sk-67{\deg}111 to Sk-67{\deg}101, accompanied by only a small variation in the OI column density. Based on the formation-dissociation equilibrium, we trace these variations to the actual density variations in the molecular gas. On the smallest spatial scale of < 0.08 pc, between Sk-67{\deg}107 and LH54-425, we find a gas density variation of a factor of 1.8. The line of sight towards LH54-425 does not follow the relatively smooth change seen from Sk-67{\deg}101 to Sk-67{\deg}111, suggesting that sub-structure might exist on a smaller spatial scale than the linear extent of our sight-lines. Our observations suggest that the detected H_2 in these six lines of sight is not necessarily physically connected, but that we are sampling molecular cloudlets with pathlengths < 0.1-1.8 pc and possibly different densities.Comment: 17 pages, 8 figures. Accepted for publication in A&

The discovery and classification of 16 supernovae at high redshifts in ELAIS-S1 - the Stockholm VIMOS Supernova Survey II

Supernova surveys can be used to study a variety of subjects such as: (i) cosmology through type Ia supernovae (SNe), (ii) star-formation rates through core-collapse SNe, and (iii) supernova properties and their connection to host galaxy characteristics. The Stockholm VIMOS Supernova Survey (SVISS) is a multi-band imaging survey aiming to detect supernovae at redshift ~0.5 and derive thermonuclear and core-collapse supernova rates at high redshift. In this paper we present the supernovae discovered in the survey along with light curves and a photometric classification into thermonuclear and core-collapse types. To detect the supernovae in the VLT/VIMOS multi-epoch images, we used difference imaging and a combination of automatic and manual source detection to minimise the number of spurious detections. Photometry for the found variable sources was obtained and careful simulations were made to estimate correct errors. The light curves were typed using a Bayesian probability method and Monte Carlo simulations were used to study misclassification. We detected 16 supernovae, nine of which had a core-collapse origin and seven had a thermonuclear origin. The estimated misclassification errors are quite small, in the order of 5%, but vary with both redshift and type. The mean redshift of the supernovae is 0.58. Additionally, we found a variable source with a very extended light curve that could possibly be a pair instability supernova.Comment: 30 pages, 15 figures, Accepted for publication in A&A. Version with high resolution images available at: http://www.astro.su.se/~jens/sviss_sne.pd

Density variations in Milky Way gas

Recent studies at optical and radio wavelengths have shown that interstellar clouds have density structures on scales smaller than 1 pc. These have been identified in form of temporal variations or differences in column density within small angular separations. Despite the number of detections with various methods, the nature and ubiquity of the small-scale structure in the ISM is not yet fully understood. Using UV absorption data from FUSE of six LMC stars with small angular separations we study these small-scale structures in the Galactic disk and IV gas at spatial scales less than 1 pc. Based on H2 and O

The outer rings of SN 1987A

Aims. We investigate the physical properties and structure of the outer rings of SN 1987A to understand their formation and evolution. Methods. We used low resolution spectroscopy from VLT/FORS1 and high resolution spectra from VLT/UVES to estimate the physical conditions in the outer rings, using nebular analysis for emission lines such as [O II], [O III], [N II], and [S II]. We also measured the velocity at two positions of the outer rings to test a geometrical model for the rings. Additionally, we used data from the HST science archives to check the evolution of the outer rings of SN 1987A for a period that covers almost 11 years. Results. We measured the flux in four different regions, two for each outer ring. We chose regions away from the two bright neighbouring stars and as far as possible from the inner ring and created light curves for the emission lines of [O III], Hα, and [N II]. The light curves display a declining behaviour, which is consistent with the initial supernova-flash powering of the outer rings. The electron density of the emitting gas in the outer rings, as estimated by nebular analysis from the [O II] and [S II] lines, is  ≲ 3 × 103 cm-3, has not changed over the last  ~15 years, and the [N II] temperature remains also fairly constant at  ~1.2 × 104 K. We find no obvious difference in density and temperature for the two outer rings. The highest density, as estimated from the decay of Hα, could be  ~5 × 103 cm-3 however, and because the decay is somewhat faster in the southern outer ring than it is in the northern, the highest density in the outer rings may be found in the southern outer ring. For an assumed distance of 50 kpc to the supernova, the distance between the supernova and the closest parts of the outer rings could be as short as  ~1.7 × 1018 cm. Interaction between the supernova ejecta and the outer rings could therefore start in less than  ~20 years. We do not expect the outer rings to show the same optical display as the equatorial ring when this happens. Instead soft X-rays should provide a better way of observing the ejecta - outer rings interaction

The Rate of Supernovae at Redshift 0.1 − 1.0 : the Stockholm VIMOS Supernova Survey IV

We present supernova rate measurements at redshift 0.1–1.5 from the Stockholm VIMOS Supernova Survey (SVISS). The sample contains 16 supernovae in total. The discovered supernovae have been classified into core collapse or thermonuclear (Ia) types based on their light curves, colour evolution and host galaxy photometric redshift. The rates we find for the core collapse supernovae are 1.25 (+2.27 +0.85 −0.97 −0.78) - with statistical and systematic errors, respectively - at z = 0.39 and 6.90 (+5.24 +3.04 −3.25 −2.14) at z = 0.73. For the Ia supernovae the rates are 2.02 (+1.57 +0.53−0.96 −0.57) at z = 0.39 and 1.03 (+0.92 +0.31−0.54 −0.36) at z = 0.80. All of these rate estimates have been corrected for host galaxy extinction. Using Monte Carlo simulations we make a thorough study of the systematic effects from assumptions made when calculating the rates and find that the most important errors comes from misclassification, the assumed mix of faint and bright supernova types and uncertainties in redshift. We compare our rates to other observations, to the star formation history for core collapse rates and to different models of the delay time distribution for Ia rates. Overall, our measurements agree quite well with these other rates when using redshift-dependent corrections for extinction. We do not find any evidence of a missing fraction of core collapse supernovae.Author count:10;</p