Radiative shocks create environments for dust formation in classical novae

Abstract

Classical novae commonly show evidence of rapid dust formation within months of the outburst. However, it is unclear how molecules and grains are able to condense within the ejecta, given the potentially harsh environment created by ionizing radiation from the white dwarf. Motivated by the evidence for powerful radiative shocks within nova outflows, we propose that dust formation occurs within the cool, dense shell behind these shocks. We incorporate a simple molecular chemistry network and classical nucleation theory with a model for the thermodynamic evolution of the post-shock gas, in order to demonstrate the formation of both carbon and forsterite (Mg 2 SiO 4 ) grains. The high densities due to radiative shock compression (n ∼ 10 14 cm −3 ) result in CO saturation and rapid dust nucleation. Grains grow efficiently to large sizes 0.1µm, in agreement with IR observations of dust-producing novae, and with total dust masses sufficient to explain massive extinction events such as V705 Cas. As in dense stellar winds, dust formation is CO-regulated, with carbon-rich flows producing carbon-rich grains and oxygen-rich flows primarily forming silicates. CO is destroyed by non-thermal particles accelerated at the shock, allowing additional grain formation at late times, but the efficiency of this process appears to be low. Given observations showing that individual novae produce both carbonaceous and silicate grains, we concur with previous works attributing this bimodality to chemical heterogeneity of the ejecta. Nova outflows are diverse and inhomogeneous, and the observed variety of dust formation events can be reconciled by different abundances, the range of shock properties, and the observer viewing angle. The latter may govern the magnitude of extinction, with the deepest extinction events occurring for observers within the binary equatorial plane.

A Detailed Observational Analysis of V1324 Sco, the Most Gamma-Ray-luminous Classical Nova to Date

“…For example, there is no sign of dust formation in V959Mon , and V339Del showed signatures of dust formation at infrared wavelengths, but the event did not have a profound effect on the optical light curve (Gehrz et al 2015;Evans et al 2017). Derdzinski et al (2017) find that some of these variations could be attributable to viewing angle, if dust preferentially forms along the orbital plane, as would be expected in the geometry suggested by Chomiuk et al (2014a). In addition, Evans et al (2008) point out that novae on CO white dwarfs are more likely to produce dust than novae on ONe white dwarfs.…”

“…If the shocks in novae are dense and radiative (as predicted by Metzger et al 2014Metzger et al , 2015, then they are ideal locations for dust formation (Derdzinski et al 2017). Radiative shocks can also explain the observed gamma-ray luminosity and nondetection in X-rays (Section 5; Metzger et al 2014).…”

“…On the other hand, six other dusty novae-including V1324Sco-were observed but not detected in X-rays (V1324 Sco, V2676 Oph, V2361 Cyg, V1065 Cen, V2615 Oph, and V5579 Sgr; again, see Schwarz et al 2001). This lack of X-ray emission in dusty novae could be explained if dust is a signature of radiative shocks and cold, dense shells (Derdzinski et al 2017), which would absorb the majority of X-rays and re-emit them at optical wavelengths (Metzger et al 2014(Metzger et al , 2015.…”

“…However, for low-velocity (  v 1500 sh km s −1 ) shocks expanding into dense media, like internal shocks in novae, cooling is very efficient and drives the post-shock gas temperature toT 10 4 K (Metzger et al 2014;Derdzinski et al 2017). This makes it difficult to achieve the~-10 10 5 6 K gas necessary for the initial radio bump to be explained by thermal emission.…”

“…The fast wind then impacts the slow component and shock interaction ensues. Assuming that the shock is radiative (Metzger et al 2014), there will be a cold layer of material between the forward and reverse shocks (note that this is also the region where dust will form; Derdzinski et al 2017). We denote the mass of this cold shell as M shell , which grows in time as To determine the amount of time that the gamma-ray emission will persist-hereafter referred to as g t -we need to find the amount of time it will take for the shock to cross the initial slow component, i.e.,…”