ALBERT

Description: Using over a million and a half extragalactic spectra from the Sloan Digital Sky Survey we study the correlations of the diffuse interstellar bands (DIBs) in the Milky Way. We measure the correlation between DIB strength and dust extinction for 142 DIBs using 24 stacked spectra in the reddening range E ( B  –  V ) 〈 0.2, many more lines than ever studied before. Most of the DIBs do not correlate with dust extinction. However, we find 10 weak and barely studied DIBs with correlations that are higher than 0.7 with dust extinction and confirm the high correlation of additional five strong DIBs. Furthermore, we find a pair of DIBs, 5925.9 and 5927.5 Å, which exhibits significant negative correlation with dust extinction, indicating that their carrier may be depleted on dust. We use Machine Learning algorithms to divide the DIBs to spectroscopic families based on 250 stacked spectra. By removing the dust dependence, we study how DIBs follow their local environment. We thus obtain six groups of weak DIBs, four of which are tightly associated with C 2 or CN absorption lines.

Description: Modern time-domain surveys continuously monitor large swaths of the sky to look for astronomical variability. Astrophysical discovery in such data sets is complicated by the fact that detections of real transient and variable sources are highly outnumbered by ‘ bogus ’ detections caused by imperfect subtractions, atmospheric effects and detector artefacts. In this work, we present a machine-learning (ML) framework for discovery of variability in time-domain imaging surveys. Our ML methods provide probabilistic statements, in near real time, about the degree to which each newly observed source is an astrophysically relevant source of variable brightness. We provide details about each of the analysis steps involved, including compilation of the training and testing sets, construction of descriptive image-based and contextual features, and optimization of the feature subset and model tuning parameters. Using a validation set of nearly 30 000 objects from the Palomar Transient Factory, we demonstrate a missed detection rate of at most 7.7 per cent at our chosen false-positive rate of 1 per cent for an optimized ML classifier of 23 features, selected to avoid feature correlation and overfitting from an initial library of 42 attributes. Importantly, we show that our classification methodology is insensitive to mislabelled training data up to a contamination of nearly 10 per cent, making it easier to compile sufficient training sets for accurate performance in future surveys. This ML framework, if so adopted, should enable the maximization of scientific gain from future synoptic survey and enable fast follow-up decisions on the vast amounts of streaming data produced by such experiments.

Description: We study a sample of 11 Type II supernovae (SNe) discovered by the OGLE-IV survey. All objects have well-sampled I -band light curves, and at least one spectrum. We find that two or three of the 11 SNe have a declining light curve, and spectra consistent with other SNe II-L, while the rest have plateaus that can be as short as 70 d, unlike the 100 d typically found in nearby galaxies. The OGLE SNe are also brighter, and show that magnitude-limited surveys find SNe that are different than usually found in nearby galaxies. We discuss this sample in the context of understanding Type II SNe as a class and their suggested use as standard candles.

Description: Using over a million and a half extragalactic spectra from the Sloan Digital Sky Survey we study the correlations of the diffuse interstellar bands (DIBs) in the Milky Way. We measure the correlation between DIB strength and dust extinction for 142 DIBs using 24 stacked spectra in the reddening range E ( B  –  V ) 〈 0.2, many more lines than ever studied before. Most of the DIBs do not correlate with dust extinction. However, we find 10 weak and barely studied DIBs with correlations that are higher than 0.7 with dust extinction and confirm the high correlation of additional five strong DIBs. Furthermore, we find a pair of DIBs, 5925.9 and 5927.5 Å, which exhibits significant negative correlation with dust extinction, indicating that their carrier may be depleted on dust. We use Machine Learning algorithms to divide the DIBs to spectroscopic families based on 250 stacked spectra. By removing the dust dependence, we study how DIBs follow their local environment. We thus obtain six groups of weak DIBs, four of which are tightly associated with C 2 or CN absorption lines.

Description: We present an investigation of the optical spectra of 264 low-redshift ( z 〈 0.2) Type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory, an untargeted transient survey. We focus on velocity and pseudo-equivalent width measurements of the Si ii 4130, 5972, and 6355 Å lines, as well those of the Ca ii near-infrared (NIR) triplet, up to +5 days relative to the SN B -band maximum light. We find that a high-velocity component of the Ca ii NIR triplet is needed to explain the spectrum in ~95 per cent of SNe Ia observed before –5 days, decreasing to ~80 per cent at maximum. The average velocity of the Ca ii high-velocity component is ~8500 km s –1 higher than the photospheric component. We confirm previous results that SNe Ia around maximum light with a larger contribution from the high-velocity component relative to the photospheric component in their Ca ii NIR feature have, on average, broader light curves and lower Ca ii NIR photospheric velocities. We find that these relations are driven by both a stronger high-velocity component and a weaker contribution from the photospheric Ca ii NIR component in broader light curve SNe Ia. We identify the presence of C ii in very-early-time SN Ia spectra (before –10 days), finding that 〉40 per cent of SNe Ia observed at these phases show signs of unburnt material in their spectra, and that C ii features are more likely to be found in SNe Ia having narrower light curves.

Description: What are Type II-Linear supernovae (SNe II-L)? This class, which has been ill defined for decades, now receives significant attention – both theoretically, in order to understand what happens to stars in the ~15–25 M range, and observationally, with two independent studies suggesting that they cannot be cleanly separated photometrically from the regular hydrogen-rich SNe II-P characterized by a marked plateau in their light curve. Here, we analyse the multiband light curves and extensive spectroscopic coverage of a sample of 35 SNe II and find that 11 of them could be SNe II-L. The spectra of these SNe are hydrogen deficient, typically have shallow Hα absorption, may show indirect signs of helium via strong O  i 7774 absorption, and have faster line velocities consistent with a thin hydrogen shell. The light curves can be mostly differentiated from those of the regular, hydrogen-rich SNe II-P by their steeper decline rates and higher luminosity, and we propose to define them based on their decline in the V band: SNe II-L decline by more than 0.5 mag from peak brightness by day 50 after explosion. Using our sample we provide template light curves for SNe II-L and II-P in four photometric bands.

Description: Using over a million and a half extragalactic spectra we study the properties of the mysterious diffuse interstellar bands (DIBs) in the Milky Way. These data provide us with an unprecedented sampling of the skies at high Galactic latitude and low dust column density. We present our method, study the correlation of the equivalent width of eight DIBs with dust extinction and with a few atomic species, and the distribution of four DIBs – 5780.6, 5797.1, 6204.3, and 6613.6 Å – over nearly 15 000 deg 2 . As previously found, DIBs strengths correlate with extinction and therefore inevitably with each other. However, we show that DIBs can exist even in dust-free areas. Furthermore, we find that the DIBs correlation with dust varies significantly over the sky. DIB under- or overdensities, relative to the expectation from dust, are often spread over hundreds of square degrees. These patches are different for the four DIBs, showing that they are unlikely to originate from the same carrier, as previously suggested.

Description: Three lines of evidence indicate that in the most common type of core-collapse supernovae, the energy deposited in the ejecta by the exploding core is approximately proportional to the progenitor mass cubed. This result stems from an observed uniformity of light-curve plateau duration, a correlation between mass and ejecta velocity, and the known correlation between luminosity and velocity. This result ties in analytical and numerical models together with observations, providing us with clues as to the mechanism via which the explosion of the core deposits a small fraction of its energy into the hurled envelope.

Description: How can we discover objects we did not know existed within the large data sets that now abound in astronomy? We present an outlier detection algorithm that we developed, based on an unsupervised Random Forest. We test the algorithm on more than two million galaxy spectra from the Sloan Digital Sky Survey and examine the 400 galaxies with the highest outlier score. We find objects which have extreme emission line ratios and abnormally strong absorption lines, objects with unusual continua, including extremely reddened galaxies. We find galaxy–galaxy gravitational lenses, double-peaked emission line galaxies and close galaxy pairs. We find galaxies with high ionization lines, galaxies that host supernovae and galaxies with unusual gas kinematics. Only a fraction of the outliers we find were reported by previous studies that used specific and tailored algorithms to find a single class of unusual objects. Our algorithm is general and detects all of these classes, and many more, regardless of what makes them peculiar. It can be executed on imaging, time series and other spectroscopic data, operates well with thousands of features, is not sensitive to missing values and is easily parallelizable.

Description: We present optical photometry and spectroscopy of the broad-lined Type Ic supernova (SN Ic-BL) PTF10qts, which was discovered as part of the Palomar Transient Factory. The SN was located in a dwarf galaxy of magnitude r  = 21.1 at a redshift z  = 0.0907. We find that the R -band light curve is a poor proxy for bolometric data and use photometric and spectroscopic data to construct and constrain the bolometric light curve. The derived bolometric magnitude at maximum light is M bol  = –18.51 ± 0.2 mag, comparable to that of SN 1998bw ( M bol  = –18.7 mag) which was associated with a gamma-ray burst (GRB). PTF10qts is one of the most luminous SNe Ic-BL observed without an accompanying GRB. We estimate the physical parameters of the explosion using data from our programme of follow-up observations, finding that it produced a larger mass of radioactive nickel compared to other SNe Ic-BL with similar inferred ejecta masses and kinetic energies. The progenitor of the event was likely an ~20 M star.