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Register a new userLight Echoes of Transients and Variables in the Local Universe
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Astronomical light echoes, the time-dependent light scattered by dust in the vicinity of varying objects, have been recognized for over a century. Initially, their utility was thought to be confined to mapping out the three-dimensional distribution of interstellar dust. Recently, the discovery of spectroscopically-useful light echoes around centuries-old supernovae in the Milky Way and the Large Magellanic Cloud has opened up new scientific opportunities to exploit light echoes. In this review, we describe the history of light echoes in the local Universe and cover the many new developments in both the observation of light echoes and the interpretation of the light scattered from them. Among other benefits, we highlight our new ability to spectroscopically classify outbursting objects, to view them from multiple perspectives, to obtain a spectroscopic time series of the outburst, and to establish accurate distances to the source event. We also describe the broader range of variable objects whose properties may be better understood from light echo observations. Finally, we discuss the prospects of new light echo techniques not yet realized in practice.
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Light echoes, light from a variable source scattered off dust, have been observed for over a century. The recent discovery of light echoes around centuries-old supernovae in the Milky Way and the Large Magellanic Cloud have allowed the spectroscopic characterization of these events, even without contemporaneous photometry and spectroscopy using modern instrumentation. Here we review the recent scientific advances using light echoes of ancient and historic transients, and focus on our latest work on SN 1987As and Eta Carinaes light echoes.
For over a century, light echoes have been observed around variable stars and transients. The discovery of centuries-old light echoes from supernovae in the Large Magellanic Cloud has allowed the spectroscopic characterization of these events using modern instrumentation, even in the complete absence of any visual record of those events. Here we review the pivotal role the Blanco 4m telescope played in these discoveries.
We analyze a complete spectroscopic sample of galaxies ($sim$600,000 ) drawn from Sloan Digital Sky Survey (SDSS, DR7) to look for evidence of galactic winds in the local Universe. We focus on the shape of the [OIII]$lambda$5007 emission line as a tracer of ionizing gas outflows. We stack our spectra in a fine grid of star formation rate (SFR) and stellar mass to analyze the dependence of winds on the position of galaxies in the SFR versus mass diagram. We do not find any significant evidence of broad and shifted [OIII]$lambda$5007 emission line which we interpret as no evidence of outflowing ionized gas in the global population. We have also classified these galaxies as star-forming or AGN dominated according to their position in the standard BPT diagram. We show how the average [OIII]$lambda$5007 profile changes as function of nature of the dominant ionizing source. We find that in the star-forming dominated source the oxygen line is symmetric and governed by the gravitational potential well. The AGN or composite AGN$setminus$star-formation activity objects, in contrast, display a prominent and asymmetric profile that can be well described by a broad gaussian component that is blue-shifted from a narrow symmetric core. In particular, we find that the blue wings of the average [OIII]$lambda$5007 profiles are increasingly prominent in the LINERs and Seyfert galaxies. We conclude that, in the low-redshift Universe, pure star-formation activity does not seem capable of driving ionized-gas outflows, while, the presence of optically selected AGN seems to play a primary role to drive such winds. We discuss the implications of these results for the role of the quenching mechanism in the present day Universe.
We present a measurement of the volumetric rate of `calcium-rich optical transients in the local universe, using a sample of three events from the Palomar Transient Factory (PTF). This measurement builds on a detailed study of the PTF transient detection efficiencies, and uses a Monte Carlo simulation of the PTF survey. We measure the volumetric rate of calcium-rich transients to be higher than previous estimates: $1.21^{+1.13}_{-0.39}times10^{-5}$ events yr$^{-1}$ Mpc$^{-3}$. This is equivalent to 33-94% of the local volumetric type Ia supernova rate. This calcium-rich transient rate is sufficient to reproduce the observed calcium abundances in galaxy clusters, assuming an asymptotic calcium yield per calcium-rich event of ~0.05$mathrm{M}_{odot}$. We also study the PTF detection efficiency of these transients as a function of position within their candidate host galaxies. We confirm as a real physical effect previous results that suggest calcium-rich transients prefer large physical offsets from their host galaxies.
Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron and on carbon, nitrogen, and oxygen. LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.
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