Luminous red novae (LRNe) are transients characterized by low luminosities and expansion velocities, and are associated with mergers or common envelope ejections in stellar binaries. ...Intermediate-luminosity red transients (ILRTs) are an observationally similar class with unknown origins, but generally believed to either be electron capture supernovae (ECSN) in super-AGB stars, or outbursts in dusty luminous blue variables (LBVs). In this paper, we present a systematic sample of 8 LRNe and 8 ILRTs detected as part of the Census of the Local Universe (CLU) experiment on the Zwicky Transient Facility (ZTF). The CLU experiment spectroscopically classifies ZTF transients associated with nearby (\(<150\) Mpc) galaxies, achieving 80% completeness for m\(_{r}<20\)\,mag. Using the ZTF-CLU sample, we derive the first systematic LRNe volumetric-rate of 7.8\(^{+6.5}_{-3.7}\times10^{-5}\) Mpc\(^{-3}\) yr\(^{-1}\) in the luminosity range \(-16\leq\)M$_{\rm{r}}$$\leq -11\( mag. We find that in this luminosity range, the LRN rate scales as dN/dL \)\propto L^{-2.5\pm0.3}\( - significantly steeper than the previously derived scaling of \)L^{-1.4\pm0.3}\( for lower luminosity LRNe (M\)_{V}\geq-10\(). The steeper power law for LRNe at high luminosities is consistent with the massive merger rates predicted by binary population synthesis models. We find that the rates of the brightest LRNe (M\)_{r}\leq-13\( mag) are consistent with a significant fraction of them being progenitors of double compact objects (DCOs) that merge within a Hubble time. For ILRTs, we derive a volumetric rate of \)2.6^{+1.8}_{-1.4}\times10^{-6}\( Mpc\)^{-3}\(yr\)^{-1}\( for M\)_{\rm{r}}\leq-13.5\(, that scales as dN/dL \)\propto L^{-2.5\pm0.5}\(. This rate is \)\approx1-5\%$ of the local core-collapse supernova rate, and is consistent with theoretical ECSN rate estimates.
The dividing line between gamma-ray bursts (GRBs) and ordinary stripped-envelope core-collapse supernovae (SNe) is yet to be fully understood. Observationally mapping the variety of ejecta outcomes ...(ultra-relativistic, mildly-relativistic or non-relativistic) in SNe of Type Ic with broad lines (Ic-BL) can provide a key test to stellar explosion models. However, this requires large samples of the rare Ic-BL events with follow-up observations in the radio, where fast ejecta can be probed largely free of geometry and viewing angle effects. Here, we present the results of a radio (and X-ray) follow-up campaign of 16 SNe Ic-BL detected by the Zwicky Transient Facility (ZTF). Our radio campaign resulted in 4 counterpart detections and 12 deep upper limits. None of the events in our sample is as relativistic as SN 1998bw and we constrain the fraction of SN 1998bw-like explosions to \(< 19\%\) (3\(\sigma\) Gaussian equivalent), a factor of \(\approx 2\) smaller than previously established. We exclude relativistic ejecta with radio luminosity densities in between \(\approx 5\times10^{27}\) erg s\(^{-1}\) Hz\(^{-1}\) and \(\approx 10^{29}\) erg s\(^{-1}\) Hz\(^{-1}\) at \(t\gtrsim 20\) d since explosion for \(\approx 60\%\) of the events in our sample. This shows that SNe Ic-BL similar to the GRB-associated SN 1998bw, SN 2003lw, SN 2010dh, or to the relativistic SN 2009bb and iPTF17cw, are rare. Our results also exclude an association of the SNe Ic-BL in our sample with largely off-axis GRBs with energies \(E\gtrsim 10^{50}\) erg. The parameter space of SN2006aj-like events (faint and fast-peaking radio emission) is, on the other hand, left largely unconstrained and systematically exploring it represents a promising line of future research.
We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 d ...< t1/2 < 12 d, of which 28 have blue (g-r<-0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had high-S/N predominantly featureless spectra and luminous radio emission: AT2018lug and AT2020xnd. Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad H\(\alpha\) in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (six) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 22 of the transients. All X-ray and radio observations resulted in non-detections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs, and use ZTF's SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.
Stars aligned in thin stream-like features (feathers), with widths of \(\delta\sim1-10^{\circ}\) and lengths as large as \(\Delta l\sim180^{\circ}\), have been observed towards the Anticenter of our ...Galaxy and their properties mapped in abundances and phase-space. We study their origin by analysing similar features arising in an N-body simulation of a Galactic disc interacting with a Sagittarius-like dwarf spheroidal galaxy (Sgr). By following the orbits of the particles identified as contributing to feathers backwards in time, we trace their excitation to one of Sgr's previous pericentric passages. These particles initially span a large range of phase-angles but a tight range of radii, suggesting they provide a probe of populations in distinct annuli in the outer Galactic disc. The structures are long lived and persist after multiple passages on timescales of \(\sim4 \,\rm{Gyrs}\). On the sky, they exhibit oscillatory motion that can be traced with a single orbit mapped over much of their full length and with amplitudes and gradients similar to those observed. We demonstrate how these properties of feathers may be exploited to measure the potential, its flattening, as well as infer the strength of recent potential perturbations.
We present the discovery of ZTF 21aaoryiz/SN 2021fcg -- an extremely low-luminosity Type Iax supernova. SN 2021fcg was discovered by the Zwicky Transient Facility in the star-forming galaxy IC0512 at ...a distance of \(\approx\) 27 Mpc. It reached a peak absolute magnitude of \(M_{r} =\) \(-12.66\pm0.20\) mag, making it the least luminous thermonuclear supernova discovered to date. The E(B-V) contribution from the underlying host galaxy is unconstrained. However, even if it were as large as 0.5 mag, the peak absolute magnitude would be \(M_{r} = -13.78\pm0.20\) mag -- still consistent with being the lowest luminosity SN. Optical spectra of SN 2021fcg taken at 37 and 65 days post maximum show strong Ca II, Ca II and Na I D emission and several weak Fe II emission lines. The Ca II emission in the two spectra has extremely low velocities of \(\approx 1300\) and \(1000\) km s\(^{-1}\) respectively. The spectra very closely resemble those of the very low luminosity Type Iax supernovae SN 2008ha, SN 2010ae and SN 2019gsc taken at similar phases. The peak bolometric luminosity of SN 2021fcg is \(\approx\) \(2.5^{+1.5}_{-0.3}\times10^{40}\) erg s\(^{-1}\) which is a factor of three lower than that for SN 2008ha. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M\(_{\rm{Ni}} \approx 0.8^{+0.4}_{-0.5}\times10^{-3}\) M\(_{\odot}\)). The low luminosity and nickel mass of SN 2021fcg pose a challenge to the picture that low luminosity SNe Iax originate from deflagrations of near M\(_{\rm{ch}}\) hybrid carbon-oxygen-neon white dwarfs. Instead, the merger of a carbon-oxygen and oxygen-neon white dwarf is a promising model to explain SN 2021fcg.
The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure) and A13 are stellar overdensities at estimated heliocentric distances of \(d \sim 11\) kpc and 15 kpc observed at low ...Galactic latitudes towards the anticenter of our Galaxy. While these overdensities were initially thought to be remnants of a tidally-disrupted satellite galaxy, an alternate scenario is that they are composed of stars from the Milky Way (MW) disk kicked out to their current location due to interactions between a satellite galaxy and the disk. To test this scenario, we study the stellar populations of the Monoceros Ring and A13 by measuring the number of RR Lyrae and M giant stars associated with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae stars in the two structures and measure radial velocities to compare with previously measured velocities for M giant stars in the regions of the Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars (\(f_{RR:MG}\)) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae stars (116 in the Monoceros Ring and 37 in A13) and find that both structures have very low \(f_{RR:MG}\). The results support a scenario in which stars in A13 and Mon/GASS formed in the MW disk. We discuss a possible association between Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar velocity distributions and \(f_{RR:MG}\).
We present observations of SN 2021csp, the second example of a newly-identified type of supernova (Type Icn) hallmarked by strong, narrow, P Cygni carbon features at early times. The SN appears as a ...fast and luminous blue transient at early times, reaching a peak absolute magnitude of -20 within 3 days due to strong interaction between fast SN ejecta (v ~ 30000 km/s) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow line features disappear from the spectrum 10-20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ~60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic supernovae. We place SN 2021csp in context with other fast-evolving interacting transients, and discuss various progenitor scenarios: an ultrastripped progenitor star, a pulsational pair-instability eruption, or a jet-driven fallback supernova from a Wolf-Rayet star. The fallback scenario would naturally explain the similarity between these events and radio-loud fast transients, and suggests a picture in which most stars massive enough to undergo a WR phase collapse directly to black holes at the end of their lives.
The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia ...supernovae. The rate has been previously estimated in the range of \(\approx10-300\) yr\(^{-1}\), either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (\(\approx5-10\) yr\(^{-1}\)) than these estimates, even with the advent of all-sky optical time domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time domain survey. Operating in \(J\)-band (\(\approx1.2\) \(\mu\)m) that is relatively immune to dust extinction, the extinction distribution of the PGIR sample is highly skewed to large extinction values (\(> 50\)% of events obscured by \(A_V\gtrsim5\) mag). Using recent estimates for the distribution of mass and dust in the Galaxy, we show that the observed extinction distribution of the PGIR sample is commensurate with that expected from dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null hypothesis probability \(< 0.01\)%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a \(3\pi\) time domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of \(\approx 46.0^{+12.5}_{-12.4}\) yr\(^{-1}\). Our results suggest that all-sky near-infrared time-domain surveys are well poised to uncover the Galactic nova population.
We present observations of three Core-collapse supernovae (CCSNe) in elliptical hosts, detected by the Zwicky Transient Facility Bright Transient Survey (BTS). SN 2019ape is a SN Ic that exploded in ...the main body of a typical elliptical galaxy. Its properties are consistent with an explosion of a regular SN Ic progenitor. A secondary g-band light curve peak could indicate interaction of the ejecta with circumstellar material (CSM). An H\(\alpha\)-emitting source at the explosion site suggests a residual local star formation origin. SN 2018fsh and SN 2020uik are SNe II which exploded in the outskirts of elliptical galaxies. SN 2020uik shows typical spectra for SNe II, while SN 2018fsh shows a boxy nebular H\(\alpha\) profile, a signature of CSM interaction. We combine these 3 SNe with 7 events from the literature and analyze their hosts as a sample. We present multi-wavelength photometry of the hosts, and compare this to archival photometry of all BTS hosts. Using the spectroscopically complete BTS we conclude that \(0.3\%^{+0.3}_{-0.1}\) of all CCSNe occur in elliptical galaxies. We derive star-formation rates and stellar masses for the host-galaxies and compare them to the properties of other SN hosts. We show that CCSNe in ellipticals have larger physical separations from their hosts compared to SNe Ia in elliptical galaxies, and discuss implications for star-forming activity in elliptical galaxies.
We present a public catalog of transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS), a magnitude-limited (m<19 mag in either the g or r filter) survey for extragalactic ...transients in the ZTF public stream. We introduce cuts on survey coverage, sky visibility around peak light, and other properties unconnected to the nature of the transient, and show that the resulting statistical sample is spectroscopically 97% complete at <18 mag, 93% complete at <18.5 mag, and 75% complete at <19 mag. We summarize the fundamental properties of this population, identifying distinct duration-luminosity correlations in a variety of supernova (SN) classes and associating the majority of fast optical transients with well-established spectroscopic SN types (primarily SN Ibn and II/IIb). We measure the Type Ia SN and core-collapse (CC) SN rates and luminosity functions, which show good consistency with recent work. About 7% of CC SNe explode in very low-luminosity galaxies (M_i > -16 mag), 10% in red-sequence galaxies, and 1% in massive ellipticals. We find no significant difference in the luminosity or color distributions between the host galaxies of Type II and Type Ib/c supernovae, suggesting that line-driven wind stripping does not play a major role in the loss of the hydrogen envelope from their progenitors. Future large-scale classification efforts with ZTF and other wide-area surveys will provide high-quality measurements of the rates, properties, and environments of all known types of optical transients and limits on the existence of theoretically predicted but as of yet unobserved explosions.
