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[en] We present basic statistics for all supernovae discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN) during its first year-and-a-half of operations, spanning 2013 and 2014. We also present the same information for all other bright (mV ≤ 17), spectroscopically confirmed supernovae discovered from 2014 May 1 through the end of 2014, providing a comparison to the ASAS-SN sample starting from the point where ASAS-SN became operational in both hemispheres. In addition, we present collected redshifts and near-UV through IR magnitudes, where available, for all host galaxies of the bright supernovae in both samples. This work represents a comprehensive catalogue of bright supernovae and their hosts from multiple professional and amateur sources, allowing for population studies that were not previously possible because the all-sky emphasis of ASAS-SN redresses many previously existing biases. In particular, ASAS-SN systematically finds bright supernovae closer to the centres of host galaxies than either other professional surveys or amateurs, a remarkable result given ASAS-SN's poorer angular resolution. In conclusion, this is the first of a series of yearly papers on bright supernovae and their hosts that will be released by the ASAS-SN team.
[en] In this catalogue we summarize information for all supernovae discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN) and all other bright (mpeak ≤ 17), spectroscopically confirmed supernovae discovered in 2016. We then gather the near-infrared through ultraviolet magnitudes of all host galaxies and the offsets of the supernovae from the centres of their hosts from public data bases. We illustrate the results using a sample that now totals 668 supernovae discovered since 2014 May 1, including the supernovae from our previous catalogues, with type distributions closely matching those of the ideal magnitude limited sample from Li et al. This is then the third of a series of yearly papers on bright supernovae and their hosts from the ASAS-SN team.
[en] Here, this paper presents information for all supernovae discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN) during 2015, its second full year of operations. The same information is presented for bright (mV ≤ 17), spectroscopically confirmed supernovae discovered by other sources in 2015. As with the first ASAS-SN bright supernova catalogue, we also present redshifts and near-ultraviolet through infrared magnitudes for all supernova host galaxies in both samples. Combined with our previous catalogue, this work comprises a complete catalogue of 455 supernovae from multiple professional and amateur sources, allowing for population studies that were previously impossible. This is the second of a series of yearly papers on bright supernovae and their hosts from the ASAS-SN team.
[en] SN 1961V, one of Zwicky's defining Type V supernovae (SNe), was a peculiar transient in NGC 1058 that has variously been categorized as either a true core-collapse SN leaving a black hole (BH) or neutron star (NS) remnant, or an eruption of a luminous blue variable star. The former case is suggested by its possible association with a decaying non-thermal radio source, while the latter is suggested by its peculiar transient light curve and its low initial expansion velocities. The crucial difference is that the star survives a transient eruption but not an SN. All stars identified as possible survivors are significantly fainter, Lopt ∼ 105 Lsun, than the Lopt ≅ 3 x 106 Lsun progenitor star at optical wavelengths. While this can be explained by dust absorption in a shell of material ejected during the transient, the survivor must then be present as an LIR ≅ 3 x 106 Lsun mid-infrared source. Using archival Spitzer observations of the region, we show that such a luminous mid-IR source is not present. The brightest source of dust emission is only LIR ≅ 105 Lsun and does not correspond to the previously identified candidates for the surviving star. The dust cannot be made sufficiently distant and cold to avoid detection unless the ejection energy, mass, and velocity scales are those of an SN or greater. We conclude that SN 1961V was a peculiar, but real, SN. Its peculiarities are probably due to enhanced mass loss just prior to the SN, followed by the interactions of the SN blast wave with this ejecta. This adds to the evidence that there is a population of SN progenitors that have major mass-loss episodes shortly before core collapse. The progenitor is a low metallicity, ∼1/3 solar, high-mass, MZAMS ∼> 80 Msun, star, which means either that BH formation can be accompanied by an SN or that surprisingly high-mass stars can form an NS. We also report on the mid-IR properties of the two other SNe in NGC 1058, SN 1969L, and SN 2007gr.
[en] We accurately determine a new Cepheid distance to M101 (NGC 5457) using archival Hubble Space Telescope (HST)/Advanced Camera for Surveys V and I time series photometry of two fields within the galaxy. We make a slight modification to the ISIS image subtraction package to obtain optimal differential light curves from HST data. We discovered 827 Cepheids with periods between 3 and 80 days, the largest extragalactic sample of Cepheids observed with HST by a factor of two. With this large Cepheid sample, we find that the relative distance of M101 from the Large Magellanic Cloud is ΔμLMC = 10.63 ± 0.04 (random) ± 0.06 (systematic) mag. If we use the geometrically determined maser distance to NGC 4258 as our distance anchor, the distance modulus of M101 is μ0 = 29.04 ± 0.05 (random) ± 0.18 (systematic) mag or D = 6.4 ± 0.2 (random) ± 0.5 (systematic) Mpc. The uncertainty is dominated by the maser distance estimate (±0.15 mag), which should improve over the next few years. We determine a steep metallicity dependence, γ, for our Cepheid sample through two methods, yielding γ = -0.80 ± 0.21 (random) ± 0.06 (systematic) mag dex-1 and γ = -0.72+0.22-0.25 (random) ± 0.06 (systematic) mag dex-1. We see marginal evidence for variations in the Wesenheit period-luminosity relation slope as a function of deprojected galactocentric radius. We also use the tip of the red giant branch method to independently determine the distance modulus to M101 of μ0 = 29.05 ± 0.06 (random) ± 0.12 (systematic) mag.
[en] Of the approximately 350 extrasolar planets currently known, of order 10% orbit evolved stars with radii R* ∼> 2.5 Rsun. These planets are of particular interest because they tend to orbit more massive hosts, and have been subjected to variable stellar insolation over their recent histories as their primaries evolved off the main sequence. Unfortunately, we have limited information about the physical properties of these planets, as they were all detected by the radial velocity method and none have been observed to transit. Here, we evaluate the prospects for detecting transits of planetary companions to giant stars. We show that several of the known systems have a priori transit probabilities of ∼>10%, and about one transiting system is expected for the sample of host stars with R* ≥ 2.5 Rsun. Although the transits are expected to have very small amplitudes (∼few x 10-4) and long durations (∼>50 hr), we argue that the difficulty with detecting these signals in broadband light is one of systematic errors and practicality rather than photon noise, even for modest aperture (∼1 m) telescopes. We propose a novel method that may overcome these difficulties, which uses narrowband measurements to isolate the thin ring of chromospheric emission expected at the limb of giant stars. The transit signals in these narrow bands are expected to be larger in magnitude and briefer in duration than in broadband emission, and thus alleviating many of the difficulties with transit detection in broadband emission. Finally, we point out that it may be possible to discover planetary companions to giant stars using Kepler, provided that a sufficient number of such targets are monitored.
[en] We use the recently released OGLE-III catalog of 17,692 fundamental mode RR Lyr (RRab) stars in the Large Magellanic Cloud (LMC) to investigate the structure of its stellar halo. We apply conservative cuts in period, amplitude, and magnitude to remove blends and other contamination. We use period-luminosity and period-color relations to determine distance and extinction of every star in our final sample of 9393 stars. In order to determine the scatter of our method, we compare the distributions of distances in two regions at the edges of the covered area with a central region. We determine the intrinsic line-of-sight dispersion in the center to be 0.135 mag or 3.21 kpc (FWHM of 0.318 mag or 7.56 kpc), assuming zero depth in one of the edge regions. The conservative cuts we apply reduce the derived depth significantly. Furthermore, we find that the distribution of RRab stars is deformed in the sense that stars on the eastern side are closer than on the western side. We model the RRab distribution as a triaxial ellipsoid and determine its axes ratios to be 1:2.00:3.50 with the longest axis inclined by 6 deg. from the line of sight. Another result of our analysis is an extinction map of the LMC and a map of internal reddening, which we make publicly available.
[en] Understanding the late-stage evolution of the most massive stars such as η Carinae is challenging because no true analogs of η Car have been clearly identified in the Milky Way or other galaxies. In Khan et al., we utilized Spitzer IRAC images of 7 nearby (≲ 4 Mpc) galaxies to search for such analogs, and found 34 candidates with flat or red mid-IR spectral energy distributions. Here, in Paper II, we present our characterization of these candidates using multi-wavelength data from the optical through the far-IR. Our search detected no true analogs of η Car, which implies an eruption rate that is a fraction 0.01 ≲ F ≲ 0.19 of the core-collapse supernova (ccSN) rate. This is roughly consistent with each M ZAMS ≳ 70 M ☉ star undergoing one or two outbursts in its lifetime. However, we do identify a significant population of 18 lower luminosity (log (L/L ☉) ≅ 5.5-6.0) dusty stars. Stars enter this phase at a rate that is a fraction 0.09 ≲ F ≲ 0.55 of the ccSN rate, and this is consistent with all 25 < M ZAMS < 60 M ☉ stars undergoing an obscured phase at most lasting a few thousand years once or twice. These phases constitute a negligible fraction of post-main-sequence lifetimes of massive stars, which implies that these events are likely to be associated with special periods in the evolution of the stars. The mass of the obscuring material is of order ∼M ☉, and we simply do not find enough heavily obscured stars for theses phases to represent more than a modest fraction (∼10% not ∼50%) of the total mass lost by these stars. In the long term, the sources that we identified will be prime candidates for detailed physical analysis with the James Webb Space Telescope
[en] In the single-degenerate (SD) channel of a Type Ia supernovae (SNe Ia) explosion, a main-sequence (MS) donor star survives the explosion but it is stripped of mass and shock heated. An essentially unavoidable consequence of mass loss during the explosion is that the companion must have an overextended envelope after the explosion. While this has been noted previously, it has not been strongly emphasized as an inevitable consequence. We calculate the future evolution of the companion by injecting 2-6 × 1047 erg into the stellar evolution model of a 1 M☉ donor star based on the post-explosion progenitors seen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the envelope, the companion must become significantly more luminous (10-103 L☉) for a long period of time (103-104 yr). The lack of such a luminous ''leftover'' star in the LMC supernova remnant SNR 0609-67.5 provides another piece of evidence against the SD scenario. We also show that none of the stars proposed as the survivors of the Tycho supernova, including Tycho G, could plausibly be the donor star. Additionally, luminous donors closer than ∼10 Mpc should be observable with the Hubble Space Telescope starting ∼2 yr post-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe. Thus, the SD channel is already ruled out for at least two nearby SNe Ia and can easily be tested for a number of additional ones. We also discuss similar implications for the companions of core-collapse SNe.
[en] We examine the properties of 18 long period (80-210 days) and very luminous (median absolute magnitude of MI = -7.86 and MV = -6.97) Cepheids to see if they can serve as a useful distance indicator. We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow period-luminosity (PL) relation, so in fact they are more 'standard candle' like than classical Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL relation is consistent with zero. The scatter of our sample about the WI PL relation is 0.23 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a nonzero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids' main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.