Calcium-Rich Gap Transients in the Remote Outskirts of Galaxies

Abstract

From the first two seasons of the Palomar Transient Factory, we identify three peculiar transients (PTF 09dav, PTF 10iuv, PTF 11bij) with five distinguishing characteristics: peak luminosity in the gap between novae and supernovae (M R ≈ −15.5 to −16.5 mag), rapid photometric evolution (t rise ≈ 12-15 days), large photospheric velocities (≈ 6000 to 11000 km s −1 ), early spectroscopic evolution into nebular phase (≈ 1 to 3 months) and peculiar nebular spectra dominated by Calcium. We also culled the extensive decade-long Lick Observatory Supernova Search database and identified an additional member of this group, SN 2007ke. Our choice of photometric and spectroscopic properties was motivated by SN 2005E (Perets et al. 2010. To our surprise, as in the case of SN 2005E, all four members of this group are also clearly offset from the bulk of their host galaxy. Given the well-sampled early and late-time light curves, we derive ejecta masses in the range of 0.4-0.7 M ⊙ . Spectroscopically, we find that there may be a diversity in the photospheric phase, but the commonality is in the unusual nebular spectra. Our extensive follow-up observations rule out standard thermonuclear and standard core-collapse explosions for this class of "Calcium-rich gap" transients. If the progenitor is a white dwarf, we are likely seeing a detonation of the white dwarf core and perhaps even shock-front interaction with a previously ejected nova shell. In the less likely scenario of a massive star progenitor, a nonstandard channel specific to a low-metallicity environment needs to be invoked (e.g., ejecta fallback leading to black hole formation). Detection (or the lack thereof) of a faint underlying host (dwarf galaxy, cluster) will provide a crucial and decisive diagnostic to choose between these alternatives.

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

“…Figure 14. Comparison between our synthetic spectra and the observed near-maximum spectra of the Ca-rich gap transients PTF10iuv (Kasliwal et al 2012) and SN 2005E (Perets et al 2010).…”

“…Ca-rich gap transients are typically found in remote locations from the center of their host galaxies (Kasliwal et al 2012;Lyman et al 2014Lyman et al , 2016bFoley 2015). For example, PTF10iuv was 40 kpc away from the closest host galaxy candidate (Kasliwal et al 2012).…”

“…While typical Type Ib/Ic SNe reach their peak luminosity in ∼ 20 days (e.g., Drout et al 2011;Prentice et al 2016), rapidly-evolving SN LCs rise in less than ∼ 10 days and decline quickly on a similar timescale (e.g., Poznanski et al 2010;Perets et al 2010;Kawabata et al 2010;Kasliwal et al 2010;Ofek et al 2010;Kasliwal et al 2012;Drout et al 2013Drout et al , 2014Inserra et al 2015). The simplest way to interpret the rapid LC evolution of some Type Ib/Ic SNe is that their ejecta mass is much smaller than in the more slowly evolving SNe (see also, e.g., Kleiser & Kasen 2014;Drout et al 2014;Tanaka et al 2016).…”

“…This is roughly proportional to (M 3 ej /Eej) 1/4 , where Mej is the ejecta mass and Eej is the kinetic energy (e.g., Arnett 1982). The ejecta mass of rapidly-evolving SNe is typically estimated to be ∼ 0.1 M⊙, which is an order of magnitude smaller than that of typical Type Ib/Ic SNe (e.g., Poznanski et al 2010;Kasliwal et al 2012;Drout et al 2013).…”

“…One example of rapidlyevolving SNe in this luminosity range is so-called "Ca-rich gap transients." They are optical transients whose peak luminosity lie between that of classical novae and SNe and they show strong Ca lines especially in the nebular phase (e.g., Kasliwal et al 2012). Although they are called "Carich" transients, they may not necessarily be Ca-rich.…”