N. L. J. Cox

MESS (Mass-loss of Evolved StarS), a Herschel key program

MESS (Mass-loss of Evolved StarS) is a guaranteed time key program that uses the PACS and SPIRE instruments on board the Herschel space observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB stars, planetary nebulae and red supergiants, as well as luminous blue variables, Wolf-Rayet stars and supernova remnants. In total, of order 150 objects are observed in imaging and about 50 objects inspectroscopy. This paper describes the target selection and target list, and the observing strategy. Key science projects are described, and illustrated using results obtained during Herschel’s science demonstration phase. Aperture photometry is given for the 70 AGB and post-AGB stars observed up to October 17, 2010, which constitutes the largest single uniform database of far-IR and sub-mm fluxes for late-type stars.

Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions

Aims: We study the effects of local environmental conditions affecting the diffuse interstellar band (DIB) carriers within the Upper Scorpius subgroup of the Sco OB2 association. The aim is to reveal how the still unidentified DIB carriers respond to different physical conditions prevailing in interstellar clouds, in order to shed light on the origin of the DIB carriers. Methods: We obtained optical spectra with FEROS on the ESO 1.52 m telescope at La Silla, Chile, and measured the equivalent widths of five DIBs (at 5780, 5797, 6196, 6379, and 6613 A) as well as those of absorption lines of di-atomic molecules (CH, CH+, CN) and atoms (K i, Ca i) towards 89 targets in the direction of Upper Scorpius. We construct a simple radiative transfer and chemical network model of the diffuse interstellar medium (ISM) sheet in front of Upp Sco to infer the effective radiation field. Results: By measuring the DIB and molecular spectrum of diffuse clouds towards 89 sightlines in the Upper Scorpius region, we have obtained a valuable statistical dataset that provides information on the physical conditions that influence the band strengths of the DIBs. Both the interstellar radiation field strength, IUV, and the molecular hydrogen fraction, fH2, have been derived for 55 sightlines probing the Upp Sco ISM. We discuss the relations between DIB strengths, CH and CH+ line strengths, E(B-V), IUV, and fH2. The ratio between the 5780 and 5797 A DIBs reveals a (spatial) dependence on the local environment in terms of cloud density and exposure to the interstellar radiation field, reflecting the molecular nature of these DIB carriers. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO program 63.H-0456).Tables 1, 2, and 5, and Appendices are available in electronic form at http://www.aanda.org

Multi-epoch high-resolution spectroscopy of SN 2011fe - Linking the progenitor to its environment

Context. The nearby Type Ia SN 2011fe has provided an unprecedented opportunity to derive some of the properties of the progenitor system, which is one of the key open problems in the supernova (SN) field. Aims. This study attempts to establish a link between the reasonably well known nature of the progenitor and its surrounding environment. This is done with the aim of enabling the identification of similar systems in the vast majority of the cases, when distance and epoch of discovery do not allow a direct approach. Methods. To study the circumstellar environment of SN 2011fe we have obtained high-resolution spectroscopy of SN 2011fe on 12 epochs, from 8 to 86 days after the estimated date of explosion, targeting in particular at the time evolution of Caii and Nai. Results. Three main absorption systems are identified from Caii and Nai, one associated to the Milky Way, one probably arising within a high-velocity cloud, and one most likely associated to the halo of M101. The Galactic and host galaxy reddening, deduced from the integrated equivalent widths (EW) of the Nai lines are EB V=0.011 0.002 and EB V=0.014 0.002 mag, respectively. The host galaxy absorption is dominated by a component detected at the same velocity measured from the 21-cm Hi line at the projected SN position ( 180 km s 1 ). During the 3 months covered by our observations, its EW changed by 15.6 6.5 mA. This small variation is shown to be compatible with the geometric e ects produced by the rapid SN photosphere expansion coupled to the patchy fractal structure of the ISM. The observed behavior is fully consistent with ISM properties similar to those derived for our own Galaxy, with evidences for structures on scales .100 AU. Conclusions. SN 2011fe appears to be surrounded by a ”clean” environment. The lack of blue-shifted, time-variant absorption features is fully consistent with the progenitor being a binary system with a main-sequence, or even another degenerate star.

NGC 2770: A Supernova Ib Factory?

NGC 2770 has been the host of three supernovae (SNe) of Type Ib during the last ten years, SN 1999eh, SN 2007uy, and SN 2008D. SN 2008D attracted special attention due to the serendipitous discovery of an associated X-ray transient. In this paper, we study the properties of NGC 2770 and specifically the three SN sites to investigate whether this galaxy is in any way peculiar to cause a high frequency of SNe Ib. We model the global spectral energy distribution of the galaxy from broadband data and derive a star formation and SN rate comparable to the values of the Milky Way. We further study the galaxy using longslit spectroscopy covering the major axis and the three SN sites. From the spectroscopic study we find subsolar metallicities for the SN sites, a high extinction and a moderate star formation rate. In a high-resolution spectrum, we also detect diffuse interstellar bands in the line of sight toward SN 2008D. A comparison of NGC 2770 to the global properties of a galaxy sample with high SN occurrence (>= 3 SN in the last 100 years) suggests that NGC 2770 is not particularly destined to produce such an enhancement of observed SNe. Its properties are also very different from gamma-ray burst host galaxies. Statistical considerations on SN Ib detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe detected in ten years. The high number of rare Ib SNe in this galaxy is therefore likely to be a coincidence rather than special properties of the galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is highly star-forming, has a very low mass and one of the lowest metallicities detected in the nearby universe as derived from longslit spectroscopy. In the most metal poor part, we even detect Wolf-Rayet (WR) features, which is at odds with most current models of WR stars which require high metallicities. Based on observations with the Nordic Optical Telescope, ESO proposal 080.D-0526, the GALEX and NED databases.

Interstellar atoms, molecules and diffuse bands toward SN2006X in M 100

Aims. Supernovae offer the unique possibility to probe diffuse extra-galactic sightlines via observation of the optical transitions of atoms, molecules and the diffuse interstellar bands (DIBs). Through optical spectroscopy the presence of (complex) molecules in distant galaxies can be established and used to derive local physical conditions of the interstellar medium (ISM). Methods. High resolution optical (3300–6800 A) spectra of SN2006X at different phase obtained with UVES on the VLT were reduced and analysed. Results. In addition to previously detected atomic (Na i and Ca ii) and molecular (CN) transitions we present detections of DIBs (λλ6196, 6283), diatomic molecules (CH, CH + ) and neutral atoms (Ca i) in the spectra of SN2006X taken at different phases (at 2 days before and 14 and 61 days after the brightness maximum). An analysis of the absorption profiles shows no variation between phases in the abundance, nor the central velocities (within 3σ error bars) of the (dense) gas tracers (CH, CH + and Ca i )a nd the DIBs. This is consistent with the conclusion in the literature that SN2006X exploded behind a dense interstellar cloud (inferred from strong atomic sodium and calcium lines and CN transitions) which caused strong photometric reddening but whose material was not directly affected by the supernova explosion. The CH and CN column densities correspond to a reddening of one magnitude following the Galactic correlation derived previously. The λλ6196 and 6283 lines detected in the M 100 ISM are under-abundant by factor of 2.5 to 3.5 (assuming a visual extinction of ∼2 mag) compared to the average Galactic ISM relationship. Upper limits for λλ6379 and 6613 show that these are at least a factor of seven weaker. Therefore, the Galactic DIB-reddening relation does not seem to hold in M 100, although the lower gas-to-dust ratio may further reduce this discrepancy.

A SURVEY OF DIFFUSE INTERSTELLAR BANDS IN THE ANDROMEDA GALAXY: OPTICAL SPECTROSCOPY OF M31 OB STARS

We present the largest sample to date of intermediate-resolution blue-to-red optical spectra of B-type supergiants in M31 and undertake the first survey of diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications, radial velocities, and interstellar reddenings are presented for 34 stars in three regions of M31. Based on a subset of these stars with foreground-corrected reddening E M 31 B?V ? 0.05, the strengths of the M31 DIBs are analyzed with respect to the amount of dust, ultraviolet radiation field strength, and polycyclic aromatic hydrocarbon emission flux. Radial velocities and equivalent widths are given for the ?5780 and ?6283 DIBs toward 11 stars. Equivalent widths are also presented for the following DIBs detected in three sightlines in M31: ??4428, 5705, 5780, 5797, 6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers reside in clouds at radial velocities matching those of interstellar Na I and/or H I. The relationships between DIB equivalent widths and reddening (E M 31 B?V ) are consistent with those observed in the local interstellar medium (ISM) of the Milky Way (MW). Many of the observed sightlines show DIB strengths (per unit reddening) which lie at the upper end of the range of Galactic values. DIB strengths per unit reddening are found (with 68% confidence) to correlate with the interstellar UV radiation field strength. The strongest DIBs are observed where the interstellar UV flux is lowest. The mean Spitzer 8/24 ?m emission ratio in our three fields is slightly lower than that measured in the MW, but we identify no correlation between this ratio and the DIB strengths in M31. Interstellar oxygen abundances derived from the spectra of three M31 H II regions in one of the fields indicate that the average metallicity of the ISM in that region is 12 + log [O/H] = 8.54 ? 0.18, which is approximately equal to the value in the solar neighborhood.

Properties of extragalactic dust inferred from linear polarimetry of Type Ia Supernovae

Aims. The aim of this paper is twofold: 1) to investigate the properties of extragalactic dust and compare them to what is seen in the Galaxy; 2) to address in an independent way the problem of the anomalous extinction curves reported for reddened Type Ia Supernovae (SN) in connection to the environments in which they explode. Methods. The properties of the dust are derived from the wavelength dependence of the continuum polarization observed in four reddened Type Ia SN: 1986G, 2006X, 2008fp, and 2014J. The method is based on the observed fact that Type Ia SN have a negligible intrinsic continuum polarization. This and their large luminosity makes them ideal tools to probe the dust properties in extragalactic environments. Results. All four objects are characterized by exceptionally low total-to-selective absorption ratios (RV ) and display an anomalous interstellar polarization law, characterized by very blue polarization peaks. In all cases the polarization position angle is well aligned with the local spiral structure. While SN 1986G is compatible with the most extreme cases of interstellar polarization known in the Galaxy, SN 2006X, 2008fp, and 2014J show unprecedented behaviours. The observed deviations do not appear to be connected to selection effects related to the relatively large amounts of reddening characterizing the objects in the sample. Conclusions. The dust responsible for the polarization of these four SN is most likely of interstellar nature. The polarization properties can be interpreted in terms of a significantly enhanced abundance of small grains. The anomalous behaviour is apparently associated with the properties of the galactic environment in which the SN explode, rather than with the progenitor system from which they originate. For the extreme case of SN 2014J, we cannot exclude the contribution of light scattered by local material; however, the observed polarization properties require an ad hoc geometrical dust distribution.

Herschel view of the large-scale structure in the Chamaeleon dark clouds

The Chamaeleon molecular cloud complex is one of the nearest star-forming sites encompassing three molecular clouds with a different star-formation history, from quiescent (Cha III) to actively forming stars (Cha II), and reaching the end of star-formation (Cha I). To charactize its large-scale structure, we derived column density and temperature maps using PACS and SPIRE observations from the Herschel Gould Belt Survey, and applied several tools, such as filament tracing, power-spectra, \Delta-variance, and probability distribution functions of column density (PDFs), to derive physical properties. The column density maps reveal a different morphological appearance for the three clouds, with a ridge-like structure for Cha I, a clump-dominated regime for Cha II, and an intricate filamentary network for Cha III. The filament width is measured to be around 0.12\pm0.04 pc in the three clouds, and the filaments found to be gravitationally unstable in Cha I and II, but mostly subcritical in Cha III. Faint filaments (striations) are prominent in Cha I showing a preferred alignment with the large-scale magnetic field. The PDFs of all regions show a lognormal distribution at low column densities. For higher densities, the PDF of Cha I shows a turnover indicative of an extended higher density component, culminating with a power-law tail. Cha II shows a power-law tail with a slope characteristic of gravity. The PDF of Cha III can be best fit by a single lognormal. The turbulence properties of the three regions are found to be similar, pointing towards a scenario where the clouds are impacted by large-scale processes. The magnetic field could possibly play an important role for the star-formation efficiency in the Chamaeleon clouds if proven that it can effectively channel material on Cha I, and possibly Cha II, but probably less efficiently on the quiescent Cha III cloud.

Hydrogen and helium in the spectra of Type Ia supernovae

We present predictions for hydrogen and helium emission line luminosities from circumstellar matter around Type Ia supernovae (SNe Ia) using time dependent photoionization modeling. Early high-resolution ESO/VLT optical echelle spectra of the SN Ia 2000cx were taken before and up to � 70 days after maximum to probe the existence of such narrow emission lines from the supernova. We detect no such lines, and from our modeling place an upper limit on the mass loss rate for the putative wind from the progenitor system, u M < 1.3×10 −5 M⊙ yr −1 , assuming a speed of 10 km s −1 and solar abundances for the wind. If the wind would be helium-enriched and/or faster, the upper limit on u M could be significantly higher. In the helium-enriched case, we show that the best line to constrain the mass loss would be He I λ10,830. In addition to confirming the details of interstellar Na I and Ca II absorption towards SN 2000cx as discussed by Patat et al., we also find evidence for 6613.56 u Diffuse Interstellar Band (DIB) absorption in the Milky Way. We also discuss measurements of the X-ray emission from the interaction between the supernova ejecta and the wind and we re-evaluate observations of SN 1992A obtained � 16 days after maximum by Schlegel & Petre. We find an upper limit of u M � 1.3×10 −5 M⊙ yr −1 which is significantly higher than that estimated by Schlegel & Petre. These results, together with the previous observational work on the normal SNe Ia 1994D and 2001el, disfavour a symbiotic star in the upper mass loss rate regime (so called Mira type systems) from being the likely progenitor scenario for these SNe. Our model calculations are general, and can also be used for the subclass of SNe Ia that do show circumstellar interaction, e.g., the recent PTF 11kx. To constrain hydrogen in late time spectra, we present ESO/VLT and ESO/NTT optical and infrared observations of SNe Ia 1998bu and 2000cx in the nebular phase, 251 388 days after maximum. We see no signs of hydrogen line emission in SNe 1998bu and 2000cx at these epochs, and from the absence of Hα with a width of the order � 10 3 km s −1 , we argue from modeling that the mass of such hydrogen-rich gas must be < 0.03 M⊙ for both supernovae. Comparing similar upper limits with recent models of Pan et al., it seems hydrogen-rich donors with a separation of < 5 times the radius of the donor may be ruled out for the five SNe Ia 1998bu, 2000cx, 2001el, 2005am and 2005cf. Larger separation, helium-rich donors, or a double-degenerate origin for these supernovae seems more likely. Our models have also been used to put the limit on hydrogen-rich gas in the recent SN 2011fe, and for this supernova, a double-degenerate origin seems likely.

Studying the small scale ISM structure with supernovae

Aims. In this work we explore the possibility of using the fast expansion of a type la supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months. Methods. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of Na I D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations. Results. The calculations show that isolated, small (≤100 AU) clouds with Na I column densities exceeding a few 10 11 cm -2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10 12 cm -2 on a scale of 1000 AU. Conclusions. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.