Explosions of LBV and Post-LBV Stars

In this contributed talk I presented the observational evidence for supernova (SN) explosions of stars in the luminous blue variable (LBV) and the immediate post-LBV evolutionary phases. We now have compelling indications that two recent SNe of Type II-“narrow” (IIn) were the explosions of LBVs, including the direct identification of the progenitor LBV for one of these examples. A recent SN of Type Ic exploded as a helium star, two years after the powerful LBV outburst of its progenitor. These cases were also discussed by other presenters at this Workshop in some detail. I instead focus more on another example, SN2001em, which was first identified as a Type Ib/c, but later evolved to Type IIn. I argue that the progenitor of this SN exploded as a Wolf- Rayet (WR) star, following an eruptive LBV phase. Furthermore, I suggest that two “SN impostors,” i.e., extragalactic massive stars observed to undergo pre-SN LBV eruptions (similar to η Carinae), may well have evolved to the WR phase in real time

Late-time Dust Emission from the Type IIn Supernova 1995N

Type IIn supernovae (SNe IIn) have been found to be associated with significant amounts of dust. These core-collapse events are generally expected to be the final stage in the evolution of highly massive stars, either while in an extreme red supergiant phase or during a luminous blue variable phase. Both evolutionary scenarios involve substantial pre-supernova mass loss. I have analyzed the SN IIn 1995N in MCG –02–38–017 (Arp 261), for which mid-infrared archival data obtained with the Spitzer Space Telescope in 2009 (~14.7 yr after explosion) and with the Wide-field Infrared Survey Explorer in 2010 (~15.6-16.0 yr after explosion) reveal a luminous (~2 × 10^7 L_☉) source detected from 3.4 to 24 μm. These observations probe the circumstellar material, set up by pre-SN mass loss, around the progenitor star and indicate the presence of ~0.05-0.12 M_☉ of pre-existing, cool dust at ~240 K. This is at least a factor ~10 lower than the dust mass required to be produced from SNe at high redshift, but the case of SN 1995N lends further evidence that highly massive stars could themselves be important sources of dust

Searching for Progenitors of Core-Collapse Supernovae

Identifying the massive progenitor stars that give rise to core-collapse supernovae is one of the main pursuits of supernova and stellar evolution studies. In this talk I discuss some aspects of the pursuit of these progenitor stars in ground-based and Hubble Space Telescope images.Comment: 6 pages, 4 figures, to appear in "1604-2004: Supernovae as Cosmological Lighthouses," eds. M. Turatto, W. Shea, S. Benetti and L. Zampieri, ASP Conference Serie

Mid-Infrared Circumstellar Shell Sources Discovered with Spitzer: An Obscured Population of Massive Stars?

We have discovered a large number of circular and elliptical shells around luminous central sources at 24 μm with the MIPS instrument on board the Spitzer Space Telescope. Most of these shells are not visible in the shorter wavelengths bands of IRAC or archival 2MASS and optical images. On the other hand, many of the central stars are detected in the 2MASS catalog, but lack an optical counterpart, indicating that we are dealing with a population of highly obscured objects. Our archival follow-up effort has revealed 90% of these shell sources to be previously unknown

12 New Galactic Wolf-Rayet Stars Identified via 2MASS+Spitzer/GLIMPSE

We report new results from our effort to identify obscured Wolf-Rayet stars in the Galaxy. Candidates were selected by their near-infrared (2MASS) and mid-infrared (Spitzer/GLIMPSE) color excesses, which are consistent with free-free emission from ionized stellar winds and thermal excess from hot dust. We have confirmed 12 new Wolf-Rayet stars in the Galactic disk, including 9 of the nitrogen subtype (WN), and 3 of the carbon subtype (WC); this raises the total number of Wolf-Rayet stars discovered with our approach to 27. We classify one of the new stars as a possible dust-producing WC9d+OBI colliding-wind binary, as evidenced by an infrared excess resembling that of known WC9d stars, the detection of OBI features superimposed on the WC9 spectrum, and hard X-ray emission detected by XMM-Newton. A WC8 star in our sample appears to be a member of the stellar cluster Danks 1, in contrast to the rest of the confirmed Wolf-Rayet stars that generally do not appear to reside within dense stellar clusters. Either the majority of the stars are runaways from clusters, or they formed in relative isolation. We briefly discuss prospects for the expansion and improvement of the search for Wolf-Rayet stars throughout the Milky Way Galaxy.Comment: Submitted to PASP March 12, 2009; Accepted on May 14, 200

Red Eyes on Wolf-Rayet Stars: 60 New Discoveries via Infrared Color Selection

We have spectroscopically identified 60 Galactic Wolf-Rayet (WR) stars, including 38 nitrogen types (WN) and 22 carbon types (WC). Using photometry from the Spitzer/GLIMPSE and 2MASS databases, the WRs were selected via a method we have established that exploits their unique infrared colors, which is mainly the result of excess radiation from free-free scattering within their dense ionized winds. The selection criteria has been refined since our last report, and now yields WRs at a rate of ~20% in spectroscopic follow-up of candidates that comprise a broad color space defined by the color distribution of all known WRs having B>14 mag. However, there are subregions within the broad color space which yield WRs at a rate of >50%. Cross-correlation of WR candidates with archival X-ray point-source catalogs increases the WR detection rate of the broad color space to ~40%; ten new WR X-ray sources have been found, in addition to a previously unrecognized X-ray counterpart to a known WR. The extinction values, distances, and galactocentric radii of all new WRs are calculated using the method of spectroscopic parallax. Although the majority of the new WRs have no obvious association with stellar clusters, two WC8 stars reside in a previously unknown massive-star cluster that lies near the intersection of the Scutum-Centaurus Arm and the Galaxy's bar, in which five OB supergiants were also identified. In addition, two WC and four WN stars were identified in association with the stellar clusters Danks 1 and 2. A WN9 star has also been associated with the cluster [DBS2003] 179. This work brings the total number of known Galactic WRs to 476, or ~7-8% of the total empirically estimated population. An examination of their Galactic distribution reveals a tracing of spiral arms and an enhanced WR surface density toward several massive-star formation sites (abridged).Comment: Accepted to the Astronomical Journal on May 20, 2011. Document is 39 pages, including 20 figures and 8 table

Three Concurrent Phases of Massive-Star Evolution in a Pulsar-Wind Nebula

The nebular material associated the the SNR G54.1+0.3 (hereafter G54) contains the the first reported instance of triggered star formation in the immediate vicinity of a Pulsar and its Wind Nebula (PWN). With 2MASS and Spitzer colors and followup near-IR spectroscopy, we have discovered the presence of a hot, massive and most likely evolved Be-type star among the cluster of stars hosted by the pulsar. This star has probably triggered cloud collapse and formation of at least 11 YSOs, which ring the nebula. In this unique cluster are now identified three concurrent stages of stellar evolution, from massive star birth, post-Main-Sequence transition, and stellar death