arXiv subjects

Avishay Gal-Yam

Publications and source records attributed to Avishay Gal-Yam.

At least 199 records · Page 11Linked to original sources

arXiv2011

The chemical evolution of the Galactic Bulge seen through micro-lensing events

Galactic bulges are central to understanding galaxy formation and evolution. Here we report on recent studies using micro-lensing events to obtain spectra of high resolution and moderately high signal-to-noise ratios of dwarf stars in the Galactic bulge. Normally this is not feasible for the faint turn-off stars in the Galactic bulge, but micro-lensing offers this possibility. Elemental abundance trends in the Galactic bulge as traced by dwarf stars are very similar to those seen for dwarf stars in the solar neighbourhood. We discuss the implications of the ages and metallicity distribution function derived for the micro-lensed dwarf stars in the Galactic bulge.

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arXiv2011

PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy

We present the discovery and characterisation of PTF10iya, a short-lived (dt ~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21 mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fit by a blackbody with T ~ 1-2 x 10^4 K and peak bolometric luminosity L_BB ~ 1-5 x 10^44 erg per s (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z = 0.22405 +/- 0.00006) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic "big blue bump" seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early "super-Eddington" phase of a solar-type star disrupted by a ~ 10^7 M_sun black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence.

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arXiv2011

The Progenitor of Supernova 2011dh/PTF11eon in Messier 51

We have identified a luminous star at the position of supernova (SN) 2011dh/PTF11eon, in pre-SN archival, multi-band images of the nearby, nearly face-on galaxy Messier 51 (M51) obtained by the Hubble Space Telescope with the Advanced Camera for Surveys. This identification has been confirmed, to the highest available astrometric precision, using a Keck-II adaptive-optics image. The available early-time spectra and photometry indicate that the SN is a stripped-envelope, core-collapse Type IIb, with a more compact progenitor (radius ~1e11 cm) than was the case for the well-studied SN IIb 1993J. We infer that the extinction to SN 2011dh and its progenitor arises from a low Galactic foreground contribution, and that the SN environment is of roughly solar metallicity. The detected object has absolute magnitude M_V^0 ~ -7.7 and effective temperature ~6000 K. The star's radius, ~1e13 cm, is more extended than what has been inferred for the SN progenitor. We speculate that the detected star is either an unrelated star very near the position of the actual progenitor, or, more likely, the progenitor's companion in a mass-transfer binary system. The position of the detected star in a Hertzsprung-Russell diagram is consistent with an initial mass of 17--19 Msun. The light of this star could easily conceal, even in the ultraviolet, the presence of a stripped, compact, very hot (~1e5 K), nitrogen-rich Wolf-Rayet star progenitor.

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arXiv2011

Circumstellar Material in Type Ia Supernovae via Sodium Absorption Features

Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.

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arXiv2011

SN2010jp (PTF10aaxi): A Jet-Driven Type II Supernova

We present photometry and spectroscopy of the peculiar TypeII supernova (SN) 2010jp, also named PTF10aaxi. The light curve exhibits a linear decline with a relatively low peak absolute magnitude of only -15.9, and a low radioactive decay luminosity at late times that suggests a nickel mass below 0.003 $M_{\odot}$. Spectra of SN2010jp display an unprecedented triple-peaked H$α$ line profile, showing: (1) a narrow (800 km/s) central component that suggests shock interaction with dense CSM; (2) high-velocity blue and red emission features centered at -12600 and +15400 km/s; and (3) broad wings extending from -22000 to +25000 km/s. These features persist during 100 days after explosion. We propose that this line profile indicates a bipolar jet-driven explosion, with the central component produced by normal SN ejecta and CSM interaction at mid latitudes, while the high-velocity bumps and broad line wings arise in a nonrelativistic bipolar jet. Two variations of the jet interpretation seem plausible: (1) A fast jet mixes 56Ni to high velocities in polar zones of the H-rich envelope, or (2) the reverse shock in the jet produces blue and red bumps in Balmer lines when a jet interacts with dense CSM. Jet-driven SNeII are predicted for collapsars resulting from a wide range of initial masses above 25 $M_{\odot}$ at sub-solar metallicity. This seems consistent with the SN host environment, which is either an extremely low-luminosity dwarf galaxy or very remote parts of an interacting pair of star-forming galaxies. It also seems consistent with the low 56Ni mass that may accompany black hole formation. We speculate that the jet survives to produce observable signatures because the star's H envelope was mostly stripped away by previous eruptive mass loss.

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arXiv2011

PTF10ops - a subluminous, normal-width lightcurve Type Ia supernova in the middle of nowhere

PTF10ops is a Type Ia supernova (SN Ia), whose lightcurve and spectral properties place it outside the current SN Ia subtype classifications. Its spectra display the characteristic lines of subluminous SNe Ia, but it has a normal-width lightcurve with a long rise-time, typical of normal luminosity SNe Ia. The early-time optical spectra of PTF10ops were modelled using a spectral fitting code and found to have all the lines typically seen in subluminous SNe Ia, without the need to invoke more uncommon elements. The host galaxy environment of PTF10ops is also unusual with no galaxy detected at the position of the SN down to an absolute limiting magnitude of r \geq -12.0 mag, but a very massive galaxy is present at a separation of ~148 kpc and at the same redshift as suggested by the SN spectral features. The progenitor of PTF10ops is most likely a very old star, possibly in a low metallicity environment, which affects its explosion mechanism and observational characteristics. PTF10ops does not easily fit into any of the current models of either subluminous or normal SN Ia progenitor channels.

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arXiv2011

Supernovae in the Subaru Deep Field: the rate and delay-time distribution of type Ia supernovae out to redshift 2

The type Ia supernova (SN Ia) rate, when compared to the cosmic star formation history (SFH), can be used to derive the delay-time distribution (DTD) of SNe Ia, which can distinguish among progenitor models. We present the results of a SN survey in the Subaru Deep Field (SDF). Over a period of 3 years, we have observed the SDF on 4 independent epochs with Suprime-Cam on the Subaru 8.2-m telescope, with 2 nights of exposure per epoch, in the R, i', and z' bands. We have discovered 150 SNe out to redshift z~2. Using 11 photometric bands from the observer-frame far-ultraviolet to the near-infrared, we derive photometric redshifts for the SN host galaxies (for 24 we also have spectroscopic redshifts). This information is combined with the SN photometry to determine the type and redshift distribution of the SN sample. Our final sample includes 28 SNe Ia in the range 1.0<z<1.5 and 10 in the range 1.5<z<2.0. Our SN Ia rate measurements are consistent with those derived from the HST/GOODS sample, but the overall uncertainty of our 1.5<z<2.0 measurement is a factor of 2 smaller, of 35-50%. We find that the SN Ia rate evolution levels off at 1.0<z<2.0, but shows no sign of declining. Combining our SN Ia rate measurements and those from the literature, and comparing to a wide range of possible SFHs, the best-fit DTD is a power law of the form Psi(t) ~ t^beta, with beta = -1.1 +/- 0.1 (statistical) +/- 0.17 (systematic). This result is consistent with other recent DTD measurements at various redshifts and environments, and is in agreement with a generic prediction of the double-degenerate progenitor scenario for SNe Ia. By combining the contribution to iron production from core-collapse SNe, based on the wide range of SFHs, with that from SNe Ia, calculated with the best-fit DTD, we predict that the mean present-day cosmic iron abundance is in the range Z(Fe) = (0.09-0.37) Z(Fe,Sun) (abridged).

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arXiv2011

PTF11eon/SN2011dh: Discovery of a Type IIb Supernova From a Compact Progenitor in the Nearby Galaxy M51

On May 31, 2011 UT a supernova (SN) exploded in the nearby galaxy M51 (the Whirlpool Galaxy). We discovered this event using small telescopes equipped with CCD cameras, as well as by the Palomar Transient Factory (PTF) survey, and rapidly confirmed it to be a Type II supernova. Our early light curve and spectroscopy indicates that PTF11eon resulted from the explosion of a relatively compact progenitor star as evidenced by the rapid shock-breakout cooling seen in the light curve, the relatively low temperature in early-time spectra and the prompt appearance of low-ionization spectral features. The spectra of PTF11eon are dominated by H lines out to day 10 after explosion, but initial signs of He appear to be present. Assuming that He lines continue to develop in the near future, this SN is likely a member of the cIIb (compact IIb; Chevalier and Soderberg 2010) class, with progenitor radius larger than that of SN 2008ax and smaller than the eIIb (extended IIb) SN 1993J progenitor. Our data imply that the object identified in pre-explosion Hubble Space Telescope images at the SN location is possibly a companion to the progenitor or a blended source, and not the progenitor star itself, as its radius (~10^13 cm) would be highly inconsistent with constraints from our post-explosion photometric and spectroscopic data.

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arXiv2011

Real-Time Detection and Rapid Multiwavelength Follow-up Observations of a Highly Subluminous Type II-P Supernova from the Palomar Transient Factory Survey

The Palomar Transient Factory (PTF) is an optical wide-field variability survey carried out using a camera with a 7.8 square degree field of view mounted on the 48-in Oschin Schmidt telescope at Palomar Observatory. One of the key goals of this survey is to conduct high-cadence monitoring of the sky in order to detect optical transient sources shortly after they occur. Here, we describe the real-time capabilities of the PTF and our related rapid multiwavelength follow-up programs, extending from the radio to the gamma-ray bands. We present as a case study observations of the optical transient PTF10vdl (SN 2010id), revealed to be a very young core-collapse (Type II-P) supernova having a remarkably low luminosity. Our results demonstrate that the PTF now provides for optical transients the real-time discovery and rapid-response follow-up capabilities previously reserved only for high-energy transients like gamma-ray bursts.

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arXiv2011

PTF10fqs: A Luminous Red Nova in the Spiral Galaxy Messier 99

The Palomar Transient Factory (PTF) is systematically charting the optical transient and variable sky. A primary science driver of PTF is building a complete inventory of transients in the local Universe (distance less than 200 Mpc). Here, we report the discovery of PTF10fqs, a transient in the luminosity "gap" between novae and supernovae. Located on a spiral arm of Messier 99, PTF 10fqs has a peak luminosity of Mr = -12.3, red color (g-r = 1.0) and is slowly evolving (decayed by 1 mag in 68 days). It has a spectrum dominated by intermediate-width H (930 km/s) and narrow calcium emission lines. The explosion signature (the light curve and spectra) is overall similar to thatof M85OT2006-1, SN2008S, and NGC300OT. The origin of these events is shrouded in mystery and controversy (and in some cases, in dust). PTF10fqs shows some evidence of a broad feature (around 8600A) that may suggest very large velocities (10,000 km/s) in this explosion. Ongoing surveys can be expected to find a few such events per year. Sensitive spectroscopy, infrared monitoring and statistics (e.g. disk versus bulge) will eventually make it possible for astronomers to unravel the nature of these mysterious explosions.

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arXiv2010

The Extreme Hosts of Extreme Supernovae

We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to illustrate how extreme they are. The LSN hosts appear to favor low-density regions of the galaxy CMD falling on the blue edge of the blue cloud toward the low luminosity end. From the UV-optical photometry, we estimate the star formation history of the LSN hosts. The hosts have moderately low star formation rates (SFRs) and low stellar masses (M_*) resulting in high specific star formation rates (sSFR). Compared with the larger sample, the LSN hosts occupy low-density regions of a diagram plotting sSFR versus M_* in the area having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts implies the LSNe are produced by an effect having to do with their local environment. The correlation of mass with metallicity suggests that perhaps wind-driven mass loss is the factor that prevents LSNe from arising in higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe (>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR.

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arXiv2010

Rapidly Decaying Supernova 2010X: A Candidate ".Ia" Explosion

We present the discovery, photometric and spectroscopic follow-up observations of SN 2010X (PTF 10bhp). This supernova decays exponentially with tau_d=5 days, and rivals the current recordholder in speed, SN 2002bj. SN 2010X peaks at M_r=-17mag and has mean velocities of 10,000 km/s. Our light curve modeling suggests a radioactivity powered event and an ejecta mass of 0.16 Msun. If powered by Nickel, we show that the Nickel mass must be very small (0.02 Msun) and that the supernova quickly becomes optically thin to gamma-rays. Our spectral modeling suggests that SN 2010X and SN 2002bj have similar chemical compositions and that one of Aluminum or Helium is present. If Aluminum is present, we speculate that this may be an accretion induced collapse of an O-Ne-Mg white dwarf. If Helium is present, all observables of SN 2010X are consistent with being a thermonuclear Helium shell detonation on a white dwarf, a ".Ia" explosion. With the 1-day dynamic-cadence experiment on the Palomar Transient Factory, we expect to annually discover a few such events.

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arXiv2010

Caltech Core-Collapse Project (CCCP) observations of type IIn supernovae: typical properties and implications for their progenitor stars

Type IIn Supernovae (SNe IIn) are rare events, constituting only a few percent of all core-collapse SNe, and the current sample of well observed SNe IIn is small. Here, we study the four SNe IIn observed by the Caltech Core-Collapse Project (CCCP). The CCCP SN sample is unbiased to the extent that object selection was not influenced by target SN properties. Therefore, these events are representative of the observed population of SNe IIn. We find that a narrow P-Cygni profile in the hydrogen Balmer lines appears to be a ubiquitous feature of SNe IIn. Our light curves show a relatively long rise time (>20 days) followed by a slow decline stage (0.01 to 0.15 mag/day), and a typical V-band peak magnitude of M_V=-18.4 +/- 1.0 mag. We measure the progenitor star wind velocities (600 - 1400 km/s) for the SNe in our sample and derive pre-explosion mass loss rates (0.026 - 0.12 solar masses per year). We compile similar data for SNe IIn from the literature, and discuss our results in the context of this larger sample. Our results indicate that typical SNe IIn arise from progenitor stars that undergo LBV-like mass-loss shortly before they explode.

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arXiv2010

Hubble Space Telescope Studies of Nearby Type Ia Supernovae: The Mean Maximum Light Ultraviolet Spectrum and its Dispersion

We present the first results of an ongoing campaign using the STIS spectrograph on-board the Hubble Space Telescope (HST) whose primary goal is the study of near ultraviolet (UV) spectra of local Type Ia supernovae (SNe Ia). Using events identified by the Palomar Transient Factory and subsequently verified by ground-based spectroscopy, we demonstrate the ability to locate and classify SNe Ia as early as 16 days prior to maximum light. This enables us to trigger HST in a non-disruptive mode to obtain near UV spectra within a few days of maximum light for comparison with earlier equivalent ground-based spectroscopic campaigns conducted at intermediate redshifts, z ~ 0.5. We analyze the spectra of 12 Type Ia supernovae located in the Hubble flow with 0.01 < z < 0.08. Although a fraction of our eventual sample, these data, together with archival data, already provide a substantial advance over that previously available. Restricting samples to those of similar phase and stretch, the mean UV spectrum agrees reasonably closely with that at intermediate redshift, although some differences are found in the metallic absorption features. A larger sample will determine whether these differences reflect possible sample biases or are a genuine evolutionary effect. Significantly, the wavelength-dependent dispersion, which is larger in the UV, follows similar trends to that observed at intermediate redshift and is driven, in part, by differences in the various metallic features. While the origin of the UV dispersion remains uncertain, our comparison suggests that it may reflect compositional variations amongst our sample rather than being predominantly an evolutionary effect.

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arXiv2010

Helium Shell Detonations on Low Mass White Dwarfs as a Possible Explanation for SN 2005E

Recently several type Ib supernovae (SNe; with the prototypical SN 2005E) have been shown to have atypical properties. These SNe are faint (absolute peak magnitude of ~ -15) and fast SNe that show unique composition. They are inferred to have low ejecta mass (a few tenths of a solar mass) and to be highly enriched in calcium, but poor in silicon elements and nickel. These SNe were therefore suggested to belong to a new class of calcium-rich faint SNe explosions. Their properties were proposed to be the result of helium detonations that may occur on helium accreting white dwarfs. In this paper we theoretically study the scenario of helium detonations, and focus on the results of detonations in accreted helium layers on low mass carbon-oxygen (CO) cores. We present new results from one dimensional simulations of such explosions, including their light curves and spectra. We find that when the density of the helium layer is low enough the helium detonation produces large amounts of intermediate elements, such as calcium and titanium, together with a large amount of unburnt helium. Our results suggest that the properties of calcium-rich faint SNe could indeed be consistent with the helium-detonation scenario on small CO cores. Above a certain density (larger CO cores) the detonation leaves mainly 56Ni and unburnt helium, and the predicted spectrum will unlikely fit the unique features of this class of SNe. Finally, none of our studied models reproduces the bright, fast evolving light curves of another type of peculiar SNe suggested to originate in helium detonations (SNe 1885A, 1939B and 2002bj).

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arXiv2010

The supernova delay time distribution in galaxy clusters and implications for Type-Ia progenitors and metal enrichment

Knowledge of the supernova (SN) delay time distribution (DTD) - the SN rate versus time that would follow a hypothetical brief burst of star formation - can shed light on SN progenitors and physics. We compile recent measurements of the Type-Ia SN (SN Ia) rate in galaxy clusters at redshifts z=0-1.45. Together with the observed iron-to-stellar mass ratio in clusters, which constrains the time-integrated number of SN Ia events in clusters, we recover the DTD of SNe Ia in cluster environments. The DTD peaks at the shortest time-delay interval we probe, 0<t<2.2 Gyr, with a low tail out to delays of ~10 Gyr, and is remarkably consistent with several recent DTD reconstructions based on different methods, in different environments. We test DTD models from the literature, requiring that they simultaneously reproduce the observed cluster SN rates and the observed iron-to-stellar mass ratios. A power-law DTD of the form t^{-1.2+/-0.3}, extending to a Hubble time, can satisfy both constraints. Shallower power laws, such as t^{-1/2} cannot, assuming a single DTD, and a single star-formation burst (either brief or extended) at high z. This implies 50-85% of SNe Ia explode within 1 Gyr of star formation. DTDs from double-degenerate (DD) models, which generically have ~t^{-1} shapes over a wide range of timescales, match the data, but only if their predictions are scaled up by factors of 5-10. Single degenerate (SD) DTDs always give poor fits to the data, due to a lack of delayed SNe and overall low numbers of SNe. The observations also permit a combination of two SN Ia populations - prompt (e.g. SD) SNe Ia that explode within a few Gyr of star formation, and produce about 60% of the iron mass in clusters, and a DD population that contributes the events seen at z<1.4. Our results support the existence of a DD progenitor channel for SNe Ia, if the overall predicted numbers can be suitably increased.

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arXiv2010

Core-Collapse Supernovae from the Palomar Transient Factory: Indications for a Different Population in Dwarf Galaxies

We use the first compilation of 72 core-collapse supernovae (SNe) from the Palomar Transient Factory (PTF) to study their observed subtype distribution in dwarf galaxies compared to giant galaxies. Our sample is the largest single-survey, untargeted, spectroscopically classified, homogeneous collection of core-collapse events ever assembled, spanning a wide host-galaxy luminosity range (down to M_r ~ -14 mag) and including a substantial fraction (>20%) of dwarf (M_r >= -18 mag) hosts. We find more core-collapse SNe in dwarf galaxies than expected and several interesting trends emerge. We use detailed subclassifications of stripped-envelope core-collapse SNe and find that all Type I core-collapse events occurring in dwarf galaxies are either SNe Ib or broad-lined SNe Ic (SNe Ic-BL), while "normal" SNe Ic dominate in giant galaxies. We also see a significant excess of SNe IIb in dwarf hosts. We hypothesize that in lower metallicity hosts, metallicity-driven mass loss is reduced, allowing massive stars that would have appeared as "normal" SNe Ic in metal-rich galaxies to retain some He and H, exploding as Ib/IIb events. At the same time, another mechanism allows some stars to undergo extensive stripping and explode as SNe Ic-BL (and presumably also as long-duration gamma-ray bursts). Our results are still limited by small-number statistics, and our measurements of the observed N(Ib/c)/N(II) number ratio in dwarf and giant hosts (0.25_{-0.15}^{+0.3} and 0.23_{-0.08}^{+0.11}, respectively; 1 sigma uncertainties) are consistent with previous studies and theoretical predictions. As additional PTF data accumulate, more robust statistical analyses will be possible, allowing the evolution of massive stars to be probed via the dwarf-galaxy SN population.

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arXiv2010

The Type Ia Supernova Rate in Redshift 0.5--0.9 Galaxy Clusters

Supernova (SN) rates are potentially powerful diagnostics of metal enrichment and SN physics, particularly in galaxy clusters with their deep, metal-retaining potentials and relatively simple star-formation histories. We have carried out a survey for supernovae (SNe) in galaxy clusters, at a redshift range 0.5<z<0.9, using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. We reimaged a sample of 15 clusters that were previously imaged by ACS, thus obtaining two to three epochs per cluster, in which we discovered five likely cluster SNe, six possible cluster SNe Ia, two hostless SN candidates, and several background and foreground events. Keck spectra of the host galaxies were obtained to establish cluster membership. We conducted detailed efficiency simulations, and measured the stellar luminosities of the clusters using Subaru images. We derive a cluster SN rate of 0.35 SNuB +0.17/-0.12 (statistical) \pm0.13 (classification) \pm0.01 (systematic) [where SNuB = SNe (100 yr 10^10 L_B_sun)^-1] and 0.112 SNuM +0.055/-0.039 (statistical) \pm0.042 (classification) \pm0.005 (systematic) [where SNuM = SNe (100 yr 10^10 M_sun)^-1]. As in previous measurements of cluster SN rates, the uncertainties are dominated by small-number statistics. The SN rate in this redshift bin is consistent with the SN rate in clusters at lower redshifts (to within the uncertainties), and shows that there is, at most, only a slight increase of cluster SN rate with increasing redshift. The low and fairly constant SN Ia rate out to z~1 implies that the bulk of the iron mass in clusters was already in place by z~1. The recently observed doubling of iron abundances in the intracluster medium between z=1 and 0, if real, is likely the result of redistribution of existing iron, rather than new production of iron.

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