Calcium-rich supernovae (Ca-rich SNe) are peculiar low-luminosity SNe Ib with relatively strong Ca spectral lines at ∼2 months after peak brightness. This class also has an extended projected offset ...distribution, with several members of the class offset from their host galaxies by 30–150 kpc. There is no indication of any stellar population at the SN positions. Using a sample of 13 Ca-rich SNe, we present kinematic evidence that the progenitors of Ca-rich SNe originate near the centres of their host galaxies and are kicked to the locations of the SN explosions. Specifically, SNe with small projected offsets have large line-of-sight velocity shifts as determined by nebular lines, while those with large projected offsets have no significant velocity shifts. Therefore, the velocity shifts must not be primarily the result of the SN explosion. Additionally, nearly every Ca-rich SN is hosted by a galaxy with indications of a recent merger and/or is in a dense environment. We propose a progenitor model which fits all current data: the progenitor system for a Ca-rich SN is a double white dwarf (WD) system where at least one WD has a significant He abundance. This system, through an interaction with a super-massive black hole (SMBH) is ejected from its host galaxy and the binary is hardened, significantly reducing the merger time. After 10–100 Myr (on average), the system explodes with a large physical offset. The rate for such events is significantly enhanced for galaxies which have undergone recent mergers, potentially making Ca-rich SNe new probes of both the galaxy merger rate and (binary) SMBH population.
We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant from 3.3% to 2.4%. The bulk of this improvement comes ...from new near-infrared (NIR) observations of Cepheid variables in 11 host galaxies of recent type Ia supernovae (SNe Ia), more than doubling the sample of reliable SNe Ia having a Cepheid-calibrated distance to a total of 19; these in turn leverage the magnitude-redshift relation based on ~300 SNe Ia at z< 0.15. All 19 hosts as well as the megamaser system NGC 4258 have been observed with WFC3 in the optical and NIR, thus nullifying cross-instrument zeropoint errors in the relative distance estimates from Cepheids. Other noteworthy improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC 4258, a larger sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to the LMC based on late-type detached eclipsing binaries (DEBs), HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC 4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes measured with HST/FGS, HST/WFC3 spatial scanning and/or Hipparcos, and (iv) 2 DEBs in M31. The Hubble constant from each is 72.25 + or - 2.51, 72.04 + or - 2.67, 76.18 + or - 2.37, and 74.50 + or - 3.27 km s super(-1) Mpc super(-1), respectively. Our best estimate of H sub(0)= 73.24 + or - 1.74 km s super(-1) Mpc super(-1) combines the anchors NGC 4258, MW, and LMC, yielding a 2.4% determination (all quoted uncertainties include fully propagated statistical and systematic components). This value is 3.4sigma higher than 66.93 + or - 0.62 km s super(-1) Mpc super(-1) predicted by LambdaCDM with 3 neutrino flavors having a mass of 0.06 eV and the new Planck data, but the discrepancy reduces to 2.1sigma relative to the prediction of 69.3 + or - 0.7 km s super(-1) Mpc super(-1) based on the comparably precise combination of WMAP+ACT+SPT+BAO observations, suggesting that systematic uncertainties in CMB radiation measurements may play a role in the tension. If we take the conflict between Planck high-redshift measurements and our local determination of H sub(0) at face value, one plausible explanation could involve an additional source of dark radiation in the early universe in the range of DeltaN sub(eff)approximate 0.4-1. We anticipate further significant improvements in H sub(0) from upcoming parallax measurements of long-period MW Cepheids.
This is the second paper of a series in which we present new measurements of the observed rates of supernovae (SNe) in the local Universe, determined from the Lick Observatory Supernova Search ...(LOSS). In this paper, a complete SN sample is constructed, and the observed (uncorrected for host-galaxy extinction) luminosity functions (LFs) of SNe are derived. These LFs solve two issues that have plagued previous rate calculations for nearby SNe: the luminosity distribution of SNe and the host-galaxy extinction. We select a volume-limited sample of 175 SNe, collect photometry for every object and fit a family of light curves to constrain the peak magnitudes and light-curve shapes. The volume-limited LFs show that they are not well represented by a Gaussian distribution. There are notable differences in the LFs for galaxies of different Hubble types (especially for SNe Ia). We derive the observed fractions for the different subclasses in a complete SN sample, and find significant fractions of SNe II-L (10 per cent), IIb (12 per cent) and IIn (9 per cent) in the SN II sample. Furthermore, we derive the LFs and the observed fractions of different SN subclasses in a magnitude-limited survey with different observation intervals, and find that the LFs are enhanced at the high-luminosity end and appear more 'standard' with smaller scatter, and that the LFs and fractions of SNe do not change significantly when the observation interval is shorter than 10 d. We also discuss the LFs in different galaxy sizes and inclinations, and for different SN subclasses. Some notable results are that there is not a strong correlation between the SN LFs and the host-galaxy size, but there might be a preference for SNe IIn to occur in small, late-type spiral galaxies. The LFs in different inclination bins do not provide strong evidence for extreme extinction in highly inclined galaxies, though the sample is still small. The LFs of different SN subclasses show significant differences. We also find that SNe Ibc and IIb come from more luminous galaxies than SNe II-P, while SNe IIn come from less luminous galaxies, suggesting a possible metallicity effect. The limitations and applications of our LFs are also discussed.
We re-examine the classifications of supernovae (SNe) presented in the Lick Observatory Supernova Search (LOSS) volume-limited sample with a focus on the stripped-envelope SNe. The LOSS ...volume-limited sample, presented by Leaman et al. and Li et al., was calibrated to provide meaningful measurements of SN rates in the local universe; the results presented therein continue to be used for comparisons to theoretical and modeling efforts. Many of the objects from the LOSS sample were originally classified based upon only a small subset of the data now available, however, and recent studies have both updated some subtype distinctions and improved our ability to perform robust classifications, especially for stripped-envelope SNe. We re-examine the spectroscopic classifications of all events in the LOSS volume-limited sample (180 SNe and SN impostors) and update them if necessary. We discuss the populations of rare objects in our sample including broad-lined SNe Ic, Ca-rich SNe, SN 1987A-like events (we identify SN 2005io as SN 1987A-like here for the first time), and peculiar subtypes. The relative fractions of SNe Ia, SNe II, and stripped-envelope SNe in the local universe are not affected, but those of some subtypes are. Most significantly, after discussing the often unclear boundary between SNe Ib and Ic when only noisy spectra are available, we find a higher SN Ib fraction and a lower SN Ic fraction than calculated by Li et al.: spectroscopically normal SNe Ib occur in the local universe 1.7 0.9 times more often than do normal SNe Ic.
Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One ...potential outcome of this "dynamically driven double-degenerate double-detonation" (D6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its >1000 km s−1 pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km s−1, and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is >1000 km s−1, the radial velocities of the other two stars are puzzlingly consistent with 0. The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.
We study the properties of tidal disruption event (TDE) host galaxies in the context of a catalog of ∼500,000 galaxies from the Sloan Digital Sky Survey. We explore whether selection effects can ...account for the overrepresentation of TDEs in E+A/post-starburst galaxies by creating matched galaxy samples. Accounting for possible selection effects due to black hole (BH) mass, redshift completeness, strong active galactic nucleus presence, bulge colors, and surface brightness can reduce the apparent overrepresentation of TDEs in E+A host galaxies by a factor of ∼4 (from ∼×100-190 to ∼×25-48), but cannot fully explain the preference. We find that TDE host galaxies have atypical photometric properties compared to similar, "typical" galaxies. In particular, TDE host galaxies tend to live in or near the "green valley" between star-forming and passive galaxies, and have bluer bulge colors ( Δ ( g − r ) 0.3 mag), lower half-light surface brightnesses (by ∼1 mag/arcsec2), higher Sérsic indices ( Δ n g 3 ), and higher bulge-to-total-light ratios ( Δ B T 0.5 ) than galaxies with matched BH masses. We find that TDE host galaxies appear more centrally concentrated and that all have high galaxy Sérsic indices and B/T fractions-on average in the top 10% of galaxies of the same BH mass-suggesting a higher nuclear stellar density. We identify a region in the Sérsic index and BH mass parameter space that contains ∼2% of our reference catalog galaxies but ≥ 60 % of TDE host galaxies. The unique photometric properties of TDE host galaxies may be useful for selecting candidate TDEs for spectroscopic follow-up observations in large transient surveys.
We use a sample of 121 spectroscopically normal Type Ia supernovae (SNe Ia) to show that their intrinsic color is correlated with their ejecta velocity, as measured from the blueshift of the Si II ...Delta *l6355 feature near maximum brightness, v Si II . The SN Ia sample was originally used by Wang et al. to show that the relationship between color excess and peak magnitude, which in the absence of intrinsic color differences describes a reddening law, was different for two subsamples split by v Si II (defined as 'Normal' and 'High Velocity'). We verify this result, but find that the two subsamples have the same reddening law when extremely reddened events (E(B -- V)>0.35 mag) are excluded. We also show that (1) the High-Velocity subsample is offset by ~0.06 mag to the red from the Normal subsample in the (B max -- V max)-MV plane, (2) the B max -- V max cumulative distribution functions of the two subsamples have nearly identical shapes, but the High-Velocity subsample is offset by ~0.07 mag to the red in B max -- V max, and (3) the bluest High-Velocity SNe Ia are ~0.10 mag redder than the bluest Normal SNe Ia. Together, this evidence indicates a difference in intrinsic color for the subsamples. Accounting for this intrinsic color difference reduces the scatter in Hubble residuals from 0.190 mag to 0.130 mag for SNe Ia with AV 0.7 mag. The scatter can be further reduced to 0.109 mag by exclusively using SNe Ia from the Normal subsample. Additionally, this result can at least partially explain the anomalously low values of RV found in large SN Ia samples. We explain the correlation between ejecta velocity and color as increased line blanketing in the High-Velocity SNe Ia, causing them to become redder. We discuss some implications of this result, and stress the importance of spectroscopy for future SN Ia cosmology surveys, with particular focus on the design of WFIRST.
Individual highly magnified stars have been recently discovered at lookback times of more than half the age of the universe, in lensed galaxies that straddle the critical curves of massive galaxy ...clusters. Having established their detectability, it is now important to carry out systematic searches for them in order to establish their frequency, and in turn learn about the statistical properties of high-redshift stars and of the granularity of matter in the foreground deflector. Here we report the discovery of a highly magnified star at redshift z = 0.94 in a strongly lensed arc behind a Hubble Frontier Field (HFF) galaxy cluster, MACS J0416.1-2403, discovered as part of a systematic archival search. The bright transient (dubbed "Warhol") was discovered in Hubble Space Telescope data taken on 2014 September 15 and 16. The point source faded over a period of two weeks, and observations taken on 2014 September 1 show that the duration of the microlensing event was at most four weeks in total. The magnified stellar image that exhibited the microlensing peak may also exhibit slow changes over a period of years at a level consistent with that expected for microlensing by the stars responsible for the intracluster light of the cluster. Optical and infrared observations taken near peak brightness can be fit by a stellar spectrum with moderate host-galaxy extinction. A blue supergiant matches the measured spectral energy distribution near peak, implying a temporary magnification of at least several thousand. The short timescale of the event and the estimated effective temperature indicate that the lensed source is an extremely magnified star. Finally, we detect the expected counterimage of the background lensed star at an offset by ∼0 1 in a deep coaddition of HFF imaging.
We present nebular spectra of the Type Ia supernova (SN Ia) SN 2019yvq, which had a bright flash of blue and ultraviolet light after exploding, followed by a rise similar to other SNe Ia. Although SN ...2019yvq displayed several other rare characteristics, such as persistent high ejecta velocity near peak brightness, it was not especially peculiar, and if the early "excess" emission were not observed, it would likely be included in cosmological samples. The excess flux can be explained by several different physical models linked to the details of the progenitor system and explosion mechanism. Each has unique predictions for the optically thin emission at late times. In our nebular spectra, we detect strong Ca ii λλ7291, 7324 and Ca near-IR triplet emission, consistent with a double-detonation explosion. We do not detect H, He, or O i emission, predictions for some single-degenerate progenitor systems and violent white dwarf mergers. The amount of swept-up H or He is <2.8 × 10−4 and 2.4 × 10−4 M , respectively. Aside from strong Ca emission, the SN 2019yvq nebular spectrum is similar to those of typical SNe Ia with the same light-curve shape. Comparing to double-detonation models, we find that the Ca emission is consistent with a model with a total progenitor mass of 1.15 M . However, we note that a lower progenitor mass better explains the early light-curve and peak luminosity. The unique properties of SN 2019yvq suggest that thick He-shell double detonations only account for of the total "normal" SN Ia rate. The SN 2019yvq is one of the best examples yet that multiple progenitor channels appear necessary to reproduce the full diversity of "normal" SNe Ia.
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