Ronald Ballouz

We carry out a systematic exploration of the effect of pre-impact rotation on<br /> the outcomes of low-speed collisions between planetesimals modeled as<br /> gravitational aggregates. We use pkdgrav, a cosmology code adapted to<br /> collisional problems and recently enhanced with a new soft-sphere collision<br /> algorithm that includes more realistic contact forces. A rotating body has<br /> lower effective surface gravity than a non-rotating one and therefore might<br /> suffer more mass loss as the result of a collision. What is less well<br /> understood, however, is whether rotation systematically increases mass loss on<br /> average regardless of the impact trajectory. This has important implications<br /> for the efficiency of planet formation via planetesimal growth, and also more<br /> generally for the determination of the impact energy threshold for catastrophic<br /> disruption (leading to the largest remnant retaining 50% of the original mass),<br /> as this has generally only been evaluated for non-spinning bodies. We find that<br /> for most collision scenarios, rotation lowers the threshold energy for<br /> catastrophic dispersal. For head-on collisions, we develop a semi-analytic<br /> description of the change in the threshold description as a function of the<br /> target&#039;s pre-impact rotation rate, and find that these results are consistent<br /> with the &quot;universal law&quot; of catastrophic disruption developed by Leinhardt &amp;<br /> Stewart. Using this approach, we introduce re-scaled catastrophic disruption<br /> variables that take into account the interacting mass fraction of the target<br /> and the projectile in order to translate oblique impacts into equivalent<br /> head-on collisions.

We present the results of a multi-component synthetic spectral analysis of<br /> the archival far ultraviolet spectra of several key nova-like variables<br /> including members of the SW Sex, RW Tri, UX UMa and VY Scl subclasses: KR Aur,<br /> RW Tri, V825 Her, V795 Her, BP Lyn, V425 Cas and HL Aqr. Accretion rates as<br /> well as the possible flux contribution of the accreting white dwarf are<br /> included in our analysis. Except for RW Tri which has a reliable trigonometric<br /> parallax, we computed the distances to the nova-like systems using the method<br /> of Knigge (2006). Our analysis of seven archival IUE spectra of RW Tri at its<br /> parallax distance of 341 pc consistently indicates a low mass (0.4Msun) white<br /> dwarf and an average accretion rate, 6.3 E-9Msun/yr. For KR Aur, we estimate<br /> that the white dwarf has Teff=29,000K, log g = 8.4 and contributes 18% of the<br /> FUV flux while an accretion disk with accretion rate of 3 E-10Msun/yr at an<br /> inclination of 41 degrees, contributes the remainder. We find that an accretion<br /> disk dominates the far UV spectrum of V425 Cas but a white dwarf contributes<br /> non-negligibly with approximately 18% of the FUV flux. For the two high state<br /> nova-likes, HL Aqr and V825 Her, their accretion disks totally dominate with 1<br /> E-9Msun/yr and 3 E-9Msun/yr, respectively. For BP Lyn we find an accretion rate<br /> of 1 E-8Msun/yr while for V795 Her, we find an accretion rate of 1 E-10Msun/yr.<br /> We discuss the implications of our results for the evolutionary status of<br /> nova-like variables.

We obtained Hubble STIS spectra of three nova-like variables: V751 Cygni,<br /> V380 Oph, and - the only confirmed nova-like variable known to be below the<br /> period gap - BK Lyn. In all three systems, the spectra were taken during high<br /> optical brightness state, and a luminous accretion disk dominates their far<br /> ultraviolet (FUV) light. We assessed a lower limit of the distances by applying<br /> the infrared photometric method of \citet{Knigge2006}. Within the limitations<br /> imposed by the poorly known system parameters (such as the inclination, white<br /> dwarf mass, and the applicability of steady state accretion disks) we obtained<br /> satisfactory fits to BK Lyn using optically thick accretion disk models with an<br /> accretion rate of $\dot{M} = 1\times10^{-9} M_{\odot}$ yr$^{-1}$ for a white<br /> dwarf mass of $M_{wd} = 1.2 M_{\odot}$ and $\dot{M} = 1 \times 10^{-8}<br /> M_{\odot}$ yr$^{-1}$ for $M_{wd} = 0.4 M_{\odot}$. However, for the VY Scl-type<br /> nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable<br /> to obtain satisfactory synthetic spectral fits to the high state FUV spectra<br /> using optically thick steady state accretion disk models. The lack of FUV<br /> spectra information down to the Lyman limit hinders the extraction of<br /> information about the accreting white dwarf during the high states of these<br /> nova-like systems.