Oct 022016

By Ori Fox (STScI)

Core-collapse supernovae (SNe) are the explosions of massive stars (>8 Msun) that reach the end of their lifetime. No longer able to radiatively support themselves by nuclear core burning after depleting their fuel, the stars collapse and release gravitational energy that rips apart the star entirely. The resulting explosions exhibit differences in their spectra and light curves that can be grouped into one of several subclasses.

From a theoretical perspective, these differences once seemed straightforward. Single star models indicate that the strength of a stellar wind increases as a function of the star’s initial mass and metallicity (Heger et al. 2003). In turn, stronger winds can remove more of a star’s outer envelope, resulting in the distribution of observed SN subclasses. Accordingly, a Type II SN has hydrogen in the spectrum, suggesting a lower mass (~8-25 Msun) red supergiant (RSG) progenitor. In contrast, a Type Ic SN has neither hydrogen nor helium in its spectrum, suggesting a higher mass (>40 Msun) progenitor.

Direct images of the individual stars before they explode provide the strongest observational constraints, but are difficult to obtain because they require deep, high-resolution, multi-color, pre-explosion imaging. Before the Hubble Space Telescope (HST) was launched, one of the few progenitors directly observed was the progenitor to SN 1987A in the Large Magellanic Cloud (LMC) at just 0.05 Mpc. The Cerro Tololo Inter-American 4-meter telescope obtained several images of the LMC between 1974 and 1983 (Walborn et al. 1987). The direct observations showed a progenitor consistent with a blue supergiant, which contradicted most stellar evolution theory and set the field on the course it is still on today.

AAT 50. The field of supernova 1987A, before and after (March, 1

Figure 1: The famous SN 1987 both before (right) and during (left) the explosion. The exploding star, named Sanduleak -69deg 202, was a blue supergiant.

Ground-based imaging is only sufficient for detecting progenitors out to 1-2 Mpc. HST extended this range out to about 20 Mpc. Cost and time, however, prohibit HST from obtaining pre-explosion imaging of the thousands of galaxies within this volume. Instead, these data must be obtained serendipitously via other science programs. The number of galaxies with pre-explosion imaging has grown steadily since HST was launched in 1990. With only a few SNe within this volume each year, a statistically significant sample of SNe with corresponding HST pre-explosion images was not accumulated until the mid-2000s (Smartt 2009). As predicted by the theory, Type II SNe had RSG progenitors. The most mystifying result, however, was the fact that the Type I SNe (i.e., those without hydrogen) had no confirmed massive (and thereby luminous) star progenitors, even to very deep limits.

The solution to this mystery is still not solved but may involve binary star progenitor systems, which are now known to account for ~75% of massive star systems (Sana et al. 2012). As opposed to single stars systems, where stars lose their envelopes in their winds, a binary companion star can remove the outer envelope of the primary via tidal stripping. This process allows for increased mass-loss from lower mass, less luminous stars that may evade detection in pre-explosion imaging. This scenario has long been preferred for a specific subclass referred to as the Type IIb (i.e., a hybrid of the Type II and Ib subclass) since most, but not all, of the outer Hydrogen envelope is removed.


Figure 2: This illustration shows the key steps in the evolution of a Type IIb supernova. Panel 1: Two very hot stars orbit about each other in a binary system. Panel 2: The slightly more massive member of the pair evolves into a bloated red giant and spills the hydrogen in its outer envelope onto the companion star. Panel 3: The more massive star explodes as a supernova. Panel 4: The companion star survives the explosion. Because it has locked up most of the hydrogen in the system, it is a larger and hotter star than when it was born. The fireball of the supernova fades. (Credit: NASA, ESA, and A. Feild (STScI))

While the primary (i.e., exploding) star in the binary system may be too faint to be detected in the pre explosion imaging, the companion star may be bright enough to test the binary hypothesis. As the primary star loses mass, the companion will gain mass and become more luminous and blue. Despite these changes, detecting the companion star in a binary system is not straightforward. The stellar spectrum of the companion will peak towards the ultraviolet (UV). Since most serendipitous pre-explosion imaging does not consist of UV observations, a UV search for the companion must occur only once the SN has faded. To date, a companion star has only been observed in a single instance for the Type IIb SN 1993J in M81 at just 3.5 Mpc (Maund et al. 2004, Fox et al. 2014).


Figure 3: This is an artist’s rendition of supernova 1993J, an exploding star in the galaxy M81 whose light reached us 21 years ago. The supernova originated in a binary system where one member was a massive star that exploded after siphoning most of its hydrogen envelope to its companion star. After two decades, astronomers have at last identified the blue helium-burning companion star, seen at the center of the expanding nebula of debris from the supernova. HST identified the UV glow of the surviving companion embedded in the fading glow of the supernova. (Credit: NASA, ESA, and G. Bacon (STScI))

The future of progenitor detections lies with HST and the James Webb Space Telescope (JWST). HST offers UV-sensitive instruments that allow us to search for the binary companions to these stripped envelope SNe. JWST will offer more than 7 times the light collecting area than HST. While JWST lacks UV capabilities necessary for companion star searches, it will increase the sensitivity to primary stars that peak at redder wavelengths. This increased sensitivity will not only provide stronger constraints on the progenitors, but it will allow progenitor searches to extend out to larger distances, thereby increasing the search volume and sample size. These new progenitors discoveries will have direct implications on our understanding of star formation, stellar evolution models, and mass loss processes.


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Aug 012016

By Anthony Marston, European Space Agency/STScI

What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are believed to be evolved massive stars that initially started their lives with masses of  > 20 Msun. With such high masses, they evolve very quickly to the WR state from high-mass hydrogen burning O stars in 1-2Myr. Currently, evidence suggests that the majority of WR stars are either in or affected by having been in relative close binaries, that can affect their evolution.

There are several evolutionary paths and theories as to the evolutionary direction of WR stars. It is postulated that different evolutionary paths exist depending on the how much initial mass exists above 20 Msun, as well as whether they are single or binary stars. For most WR stars, a mass-loss phase of a few tens of thousands of years probably occurs. Evidence for this is seen in the nitrogen-enriched ejecta nebulae that are seen around many WR stars. Ejecta are believed to be associated with a slow wind phase following the fast wind of the main sequence O star phase. Once evolved to a WR star there is again a fast wind phase which can quickly interact with a slow moving ejecta nebula. But not all WR stars are seen to have ejecta.

There are three subtypes of WR stars: WN subtypes show prominent nitrogen emission lines in their spectra, WC subtypes show prominent carbon emission lines, and WO subtypes show strong high excitation oxygen emission lines. These form an apparent evolutionary sequence with the spectra showing the products of Hydrogen burning for WN stars, Helium burning for WC star spectra and higher level burnings for WO stars. WO subtype stars in the Galaxy are very rare (three are known) and probably represent a final WR evolutionary phase before becoming a supernova (probably of type Ib).

How common are they and how are they distributed in the Galaxy?

By the end of 2000 just over 200 WR stars were known in the Galaxy. Most of these were discovered in studies of clusters or serendipitously. They were shown to follow the spiral pattern of the Galaxy and showed a distribution that mimicked other Galactic star formation site indicators. Indeed, WR stars have in various ways been used as markers of very recent high-mass star formation and star formation bursts since they only live a few million years.

In his review of WR stars, Karel van der Hucht (2001) indicated that the likely population of WR stars in the Galaxy could be several thousand rather than the few hundred known. This was in part due to obvious observational restrictions, such as unseen populations on the opposite side of the Galaxy. With the advent of sensitive infrared detectors the possibility of finding distant and/or obscured WR stars became more realistic. Two approaches have been developed for finding Galactic WR stars in recent years.

The “narrow-band” approach (Shara et al. 2009) uses narrow-band images centered on strong emission lines seen in WR stars (e.g. HeII) and subtracts from them narrow-band images covering only the continuum (or broad-band infrared images). The candidates revealed are followed up with infrared and/or optical spectroscopy to confirm their nature.

The “broad-band” approach is based on the near- to mid-infrared colors which are peculiar to stars with strong winds – and in particular WR stars. Figure 1 shows how the free-free emission from the fast WR wind of the nearby WR star WR11 (g Vel) has a distinct spectral index which is substantially different from stellar photospheres leading to WR stars being overabundant in certain areas of broadband infrared (2MASS, Spitzer/IRAC, WISE) point source color-color space (see Figure 2). Even though the this approach is slightly more prone to confusion issues than the “narrow-band” method, it has a couple of advantages over the latter: the potential for picking up weak-lined WR stars or ones where lines are diluted relative to the continuum due to a massive companion or local hot dust emission. It also uses already existing infrared point source catalogs (e.g. the GLIMPSE catalog of source within | b < 1 | in the Galactic plane). As of July 2016, the total number of known Galactic WR stars is 634 (http://www.pacrowther.staff.shef.ac.uk/WRcat/).

figure1Figure 1: Spectral energy distribution of g Velorum (Williams et al. 1990) showing the excess free-free emission from the stellar wind in the infrared wavelengths as compared to photospheric emission (straight black line). The GLIMPSE catalog which used Spitzer/IRAC data will show WR stars with colors distinct from the vast majority of stars.

Our group, with core members Schuyler Van Dyk (Caltech), Pat Morris (Caltech), Jon Mauerhan (UC Berkeley) and Anthony Marston (ESA-STScI), uses the broad-band method. It was first developed by Marston in 2004 to identify candidates in ESO/SOFI infrared spectroscopic observations and it helped identify 60 new WR stars by Mauerhan et al (2011). The color selection uses data from the GLIMPSE catalog, consisting of several 10’s of million sources detected in the Galactic plane using broadband Spitzer/IRAC 3.4 – 8 mm measurements combined with band-merged flux data from 2MASS (broadband near-infrared JHKs). In certain studies, X-ray emission sources and, more lately, WISE point source colors have been used in identifying WR candidates.  Spectroscopic follow-up has concentrated on obtaining K-band spectra, as WR stars are typically identified by strong HeII emission lines such as the 2.189mm line. For the less reddened candidates, optical spectroscopic follow-up has also been possible.

Historically we have found that 10-15% of candidates turn out to be bona fide WR, stars while~ 85% of all candidates are emission-line stars, most often Be stars. Small numbers of O/Of stars B[e] supergiants and stars exhibiting infrared CO bandhead absorption lines have been picked up where combinations of photosphere, dust emission and free-free emission has brought objects into our infrared color space. Improvements to our color-space selection have increased the success rate of WR detections out of the candidates, notably for more reddened/distant objects where the candidate confirmation rate can go as high as 25% (see Figure 2). We are currently looking into a machine-learning capability for assessing the likelihood of an object being a WR star from color-space criteria. The ultimate goal is to be able make accurate predictions of WR numbers of different subtypes in the Galaxy.


Figure 2: Infrared color-color plots showing the candidate objects observed by Mauerhan et al (2011). The green symbols were newly discovered WN subtypes and red WC subtype stars. Blue points represent candidates that follow up spectroscopy showed were not WR stars. Grey shaded areas indicate the part of the color-color plots where 50% or more candidates were found to be WR stars.

What have WR stars taught us about high-mass star formation?

The number ratios of WR to O stars and Red Supergiant (RSG) or Luminous Blue Variable stars are key values for constraining stellar evolution theories of massive star evolution. In a simple way, ratios provide an indication of relative timescales for lifetimes. Another indication of timescales, and possibly different evolutionary links between subtypes, mass-loss phases and initial stellar masses, is the number distribution of WR subtypes, both WN and WC (plus the rare WO stars).

The distribution of WR stars (studied e.g. using the Spitzer’s GLIMPSE survey across the Galactic plane) marks star formation sites across the Galaxy and indicate likely sites of future supernovae. However, it has become clear over time that many WR stars, that are no more than a few Myr in age, appear to be found well away from the centers of star-forming clusters in the Galaxy. A projection of most of the known WR stars with secure distance shows that some WR stars also appear to be more than 100 pc above/below the plane of the Galaxy (Rosslowe & Crowther, 2015). A possible explanation of why some WR stars appear to be located away from their birth site could be the presence of fast transverse motions caused by expulsion from their cluster formation site. Another possibility could be that these stars were part of small clusters but, being much more luminous than other cluster members, they appear to be isolated. But in recent years, in the study of star-forming regions like the Cygnus OB2 cluster, we have learned of an unexpected third possible explanation.

Various studies suggest that the Cygnus OB2 cluster, being 1 Myr of age, has not evolved significantly from its original distribution. This means that the massive stars, and WR stars in particular, are near the sites where they were born. However, none of the WR stars are in the massive star cluster at the center of the Cyg OB2 association (see Figure 3), and not only that, none show evidence of bow shocks from significant transverse velocities, suggesting these stars were born in situ. We now know, from studies with Herschel, that filaments of high-density gas can extend through star-forming regions with “strings” of star-forming cores being found along them. And in fact, filaments pervade the Cyg OB2 area leading to the possibility of forming high-mass stars outside of major stellar clusters, possibly instigated to form high-mass stars through a triggering event, such as expanding gas shell collisions.


Figure 3: The Cygnus OB2 association as seen by Herschel PACS/SPIRE (colored background from Schneider et al, 2016). WR stars and Luminous Blue Variable stars (likely precursors of WR stars in stellar evolution) are found well away from the major cluster of O stars shown as white points a bit to the right of center of the field (Comeron et al 2008, Wright et al 2015).

There are therefore two possible scenarios:

  • WR stars are born in situ and away from stellar clusters (but likely within stellar associations) – which means distributed high-mass star formation occurs for some of the most massive stars probably from filaments.
  • WR stars are kicked out of stellar clusters due to dynamics of the early cluster of stars or through binary/supernova interactions, apparently affecting a large fraction of the very massive stars in the stellar cluster.

As we have seen, the study of Wolf Rayet stars has shed new light on unexpected physical processes associated to high-mass star formation. In the future, we will advance their study by: (1) Using machine-learning and improved color-selection techniques to find new WR stars and assess their distributions in the Galaxy, including in high-mass star-forming regions. (2) Pinning down number ratios of WR subtypes and other massive star types. (3) Using the GAIA catalog to get proper motions of WR stars to identify runaway stars. (4) Searching for bow shocks, in particular in the mid-infrared with WISE, as it has been found that they are particularly prominent at IR wavelengths.



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Nov 152015

By Rachel Osten, AURA Astronomer at STScI

A stellar flare is the release of energy that occurs during a magnetic reconnection event in the upper atmosphere of the star.  Flares involve the release of substantial amounts of ionizing radiation, and are the most dramatic forms of energy release that cool stars will undergo during their time on the main sequence. Stellar flares appear to be an extension of the same phenomena observed in great detail on our own star, the Sun, despite the large difference in the energies involved between the two: the largest solar flares have energies of about 1032 erg, while stellar flares can be a thousand to a million times more energetic. From the solar perspective, flares are part of a triad: the flare, a coronal mass ejection, and highly energetic particles. Even though they all contribute to space weather, the latter two are the most important for determining how damaging a solar eruptive event might be. These are also the two for which we have the least some constraints on occurrence in a stellar context.


Figure: Artist’s conception of a large flare observed on a nearby M dwarf flare star.  The flare involves all layers of the star’s atmosphere, from the photosphere to the chromosphere to the tenuous, hot corona.  These large stellar flares may be accompanied by coronal mass ejections, which would affect the stellar environment around active stars.

While stellar flares have been studied for decades — non-solar stellar flares were first described by Ejnar Hertzsprung in a 1924 article entitled “Note on a peculiar variable star or nova of short duration” — it has been the discovery of planets around other stars which has provided much recent astrophysical motivation for studying stellar flares.  Several recent papers (Segura et al. 2010, Khodachenko et al. 2007) have speculated about the possible impacts on close-in exoplanets of flares and associated events like accelerated particles and coronal mass ejections.  The results suggested that the flares themselves were not the main worrisome aspect of magnetic activity that could affect habitability; rather, it was the other “messengers”, namely the coronal mass ejection and the energetic particles, which had drastic implications for habitability. In the Segura paper, the energetic particles accompanying the flare were responsible for widespread destruction of the ozone layer in the atmosphere of a  terrestrial exoplanet in the habitable zone, which took several years to recover to the pre-flare levels. In the Khodachenko paper, the frequent coronal mass ejections acted like an enhanced stellar wind, compressing the planetary magnetosphere and exposing the exoplanet’s atmosphere to enhanced levels of ionizing radiation, which can cause atmospheric loss.

Stellar flares are a multi-wavelength phenomenon. They involve all layers of the star’s atmosphere, from the photosphere to the magnetically heated chromosphere and corona, and involve a variety of physical processes, including plasma heating and particle acceleration.  Stellar flares are typically observed in a piecemeal fashion, usually covering only one wavelength region, and for short periods of time.  Imagine how little we would know about the Sun’s flares if we were limited to observing it for only a few days every year or so. And that’s if your proposal got accepted!  Even missions like Kepler, which stared at one patch of sky for multiple years and observed many thousands of stellar flares, observed in only one wavelength region; having a way to relate the energy released in one bandpass to the energy released by a flare across all wavelength regions, the so-called bolometric flare energy, is important for an intercomparison of heterogeneously observed stellar flares.

Recent work on solar flares has taken a global view of the energetics of large solar eruptive events (Kretzschmar 2011, Emslie et al. 2012). These papers established how radiated energy is partitioned in the dominant mechanisms, namely chromospheric and transition region line emission, continuum emission from a hot blackbody presumed to form in the photosphere, and coronal emission from plasma heated to temperatures in excess of a million degrees.  This solar work has also demonstrated a near equipartition between the total amount of radiated energy (the bolometric flare energy), and the kinetic energy in the associated coronal mass ejection (Emslie et al. 2012, Drake et al. 2013).

My recent paper (Osten & Wolk 2015) unites these two concepts with application to stellar flares.  When considering the potential impact of stellar eruptive events to exoplanets, one needs to “follow the photons”; current astronomical limitations means that we have few options for direct detection of coronal mass ejections, and none for the existence of these very energetic particles expected to be produced in stellar flares as they are in solar flares.  So if we want to gain an initial grasp of the potential influence of stellar eruptive events, the easiest way to do so is to look at the flares from a holistic standpoint, and relate them to coronal mass ejections using a physically motivated way of connecting them.

In order to do the latter — that is, connect the total radiated flare energy with the coronal mass ejection’s kinetic energy — we need a way to “correct” for the flare energy in a given bandpass to the bolometric amount.  Using previously published papers of a few well-observed stellar flares, I established that these flares appear to have similar fractions of energy as aggregates of solar flares (see the Table).  So far, so good.  This is another confirmation of the basic approach, in treating solar and stellar flares as originating from the same basic physical process, despite the orders of magnitude difference in energy release. Being able to relate flares observed in one wavelength range to the total amount of radiated energy released enables a more global intercomparison of those flares.


Table: Fraction of total radiated energy in Solar and stellar flares released in particular bandpasses.  The numbers for solar flares and those for active stars are remarkably similar, and confirm the expectation that the same physical process is occurring. This energy partition allows for a better intercomparison of flares observed in different wavelength regions.

We observe that stellar flares of differing energies have an occurrence that is a power-law in frequency as a function of size.   Relating the kinetic energy of coronal mass ejections to the bolometric energy of an accompanying flare (assuming equipartition, based on solar flare studies) means that we have a way to estimate the cumulative impact of the transient mass loss that is occurring in these flare-related coronal mass ejections. To make a long story short (if you want the long story, read the paper!), we find that this cumulative effect can be large.  We examined published flare frequency distributions from a variety of different types of flaring stars: young solar-mass stars; a 70 MY young solar analog; recently observed superflaring Sun-like stars; young low mass stars; nearby hyperactive M dwarfs with flare frequency distributions measured at both optical and X-ray wavelengths; and inactive early- and mid-M dwarfs.  At the high end, the flare-related transient mass loss may be as much as three orders of magnitude higher than the present-day total solar mass loss rate of 2×10-14 Msun/yr.

So what are the implications of these findings? The main impact is on the stellar environment, not the star itself. The cumulative mass loss rate, though potentially enhanced compared to the Sun’s current total rate of mass loss, is not enough to change the course of the star’s evolution, as the total mass lost is not an appreciable fraction of the total stellar mass. As mentioned before, a high rate of coronal mass ejections means that any close-in exoplanet would be subjected to enhanced compression of its magnetosphere and resulting exposure of its atmosphere to ionizing flare radiation.  The interaction between an enhanced stellar wind and an exoplanet’s magnetic field can generate a magnetospheric field that would have a negative feedback on the planet’s internal dynamo, according to one calculation. Frequent strong CMEs could quench dynamo growth in the planet, leading to weak planetary magnetic fields and a reduced ability to protect the atmosphere from exposure to ionizing radiation. Other impacts on the stellar environment include seeding the planet-forming disk with processed stellar material, or removing material from the debris disk at a later stage in the stellar system’s evolution. Results from young solar analogs suggest that the Sun’s youth was likely marked by such frequent strong CMEs. While these results are suggestive, direct constraints on the existence and occurrence of stellar coronal mass ejections are a vital next step to a fuller understanding of how stars affect their surroundings.



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