Miniworkshop on the Astrophysics of Intermediate-Luminosity Red Transients

Howard E. Bond, bond@stsci.edu

In a famous cartoon of some years ago, by the science cartoonist S. Harris, two astronomers are in the telescope dome, pondering their new data. One says to the other, “It’s somewhere between a nova and a supernova—probably a pretty good nova.” At the time, the drawing was good for a laugh among astronomical cognoscenti who were aware of the yawning gap in maximum luminosity between the classical novae and the much brighter supernovae. Over the past several years, however, this gap has started to fill in, due to discoveries (many of them by amateur astronomers) of members of a new class of astrophysical transients. They show “pretty good” maximum luminosities, intermediate between those of classical novae and supernovae, and they usually become extremely red as their outbursts proceed over timescales of a few months. Because of several dramatic new developments in this subject, the Institute hosted a workshop on intermediate-luminosity red transients (ILRTs) in the Bahcall Auditorium, June 28–30, 2011. About 30 participants from around the world attended and presented the latest results.

In the Milky Way, members of the ILRT class include V838 Monocerotis (which produced a spectacular light echo, the subject of iconic Hubble images), V4332 Sagittarii, and the recent V1309 Scorpii. Possibly related extragalactic transients include the Andromeda red variable of 1988 (M31 RV), the 2006 optical transient in M85, SN 2008S, and the 2008 and 2010 transients in the nearby galaxy NGC 300, as well as objects now being being discovered in synoptic sky surveys, such as the Palomar Transient Factory (PTF), Pan-STARRS, and the Lick supernova searches.

It has become apparent—and this was brought out strongly at the workshop—that ILRTs arise from at least two very distinct evolutionary channels. Several groups have been arguing for the past several years that the V838 Mon outburst arose from a stellar collision or binary merger, but the evidence for this scenario was mostly indirect. The new breakthrough has come from the 2008 outburst of the Galactic ILRT V1309 Sco, which occurred in the central bulge of the Milky Way. V1309 Sco lies in a field that was monitored for several years by the OGLE microlensing project, and its progenitor was found to have been a short-period contact binary. It was even possible to detect the shortening of the orbital period of the progenitor over the course of several years before its outburst. This remarkable finding clearly establishes that catastrophic stellar mergers are responsible for at least some of the spectacular ILRT eruptions. Mergers are now the leading contender to account for the ILRT outbursts that occur in old populations, such as the 1988 outburst of M31 RV in the ~10 Gyr-old bulge population of the Andromeda galaxy.

There is a second group of ILRTs, however, which are typically about 2 magnitudes brighter at maximum than the events occurring in old populations, and which probably have a different origin. Two of these eruptions occurred in 2008 in nearby galaxies: these were the Optical Transient (OT) in NGC 300, a spiral galaxy 2 Mpc away, just outside the Local Group; and “SN” 2008S, in NGC 6946—both initially discovered by amateurs

using small telescopes. The second object was designated SN 2008S before it became apparent that it was not any kind of normal supernova. In the case of the NGC 300 OT-2008, it was obviously not a supernova from the start, and it never received an SN designation.

Fortuitously, NGC 300 OT-2008 occurred in a spiral arm of NGC 300 that had been very deeply imaged by Hubble before its outburst. New Hubble images were obtained showing the ILRT, allowing the OT to be precisely localized. Remarkably, the progenitor was not detected at optical wavelengths down to magnitude 28.5 (Figure 1). However, there were also pre-eruption images of NGC 300 obtained by the Spitzer Space Telescope, which revealed a luminous mid-IR source at the outburst site. Thus the progenitor was a heavily dust-enshrouded massive star, invisible at optical wavelengths, yet one of the brightest stars in NGC 300 in the mid-IR.

SN 2008S is a near-twin of the NGC 300 OT-2008. Its progenitor was likewise optically inconspicuous, but luminous in the mid-IR, as revealed by Spitzer. The nature of these outbursts remains uncertain, but they may be related to the also poorly understood outbursts of Luminous Blue Variables (LBVs). However, there are also advocates of these eruptions being due to the elusive electron-capture supernovae hypothesized to occur among stars lying near the initial-mass boundary between objects that end up as white dwarfs and those that explode as supernovae. And it has not been ruled out that these more luminous ILRTs could also arise from binary-star interactions.

New wide-angle surveys are now finding ILRTs at a healthy rate. An example is the 2010 outburst of PTF 10fqs, occurring in a spiral arm of the Virgo galaxy M99 (Figure 2). Furthermore—seemingly in honor of the workshop—the latest ILRT, designated PSN J17592296+0617267, was discovered in the spiral galaxy NGC 6509 on the last day of the workshop. Upcoming synoptic surveys, such as the Large Synoptic Survey Telescope, will find large numbers of ILRTs in external galaxies, allowing us to refine our knowledge of the stellar populations and nature of the progenitor objects.

Figure 1. Color rendition of a Hubble/Advanced Camera for Surveys image of the site of the NGC 300 optical transient (OT) of 2008. These images were taken in 2002 and 2006. The precise astrometric location of the OT was determined from Hubble images taken during the 2008 eruption, and lies within a few pixels of the center of the green circle (radius 3 arcsec). There is no progenitor object detected down to optical magnitude 28.5. However, pre-outburst images taken by Spitzer showed a luminous mid-IR source at the eruption location, demonstrating that the progenitor was a luminous but heavily dust-enshrouded massive star. Credit: NASA/ESA Hubble Space Telescope image courtesy Luigi Bedin (STScI).

Figure 2. Hubble/Wide Field Camera 3 image showing the 2010 intermediate-luminosity red transient PTF 10fqs in outburst in the Virgo spiral M99. The dimensions of the image are 45 × 26 arcsec, and the OT is the gold-colored object just to the upper right of center, lying somewhat outside the main spiral arm. Credit: NASA/ESA Hubble Space Telescope image courtesy Mansi Kasliwal and Shri Kulkarni (Caltech), and Howard E. Bond (STScI).