Micrometeoroids (cosmic dust with size between a few $mu$m and $sim$1 mm) dominate the annual extraterrestrial mass flux to the Earth. We investigate the range of physical processes occurring when micrometeoroids traverse the atmosphere. We compute t
he time (and altitude) dependent mass loss, energy balance, and dynamics to identify which processes determine their survival for a range of entry conditions. We develop a general numerical model for the micrometeoroid-atmosphere interaction. The equations of motion, energy, and mass balance are simultaneously solved for different entry conditions (e.g. initial radii, incident speeds and angles). Several different physical processes are taken into account in the equation of energy and in the mass balance, in order to understand their relative roles and evolution during the micrometeoroid-atmosphere interaction. In particular, to analyze the micrometeoroid thermal history we include in the energy balance: collisions with atmospheric particles, micrometeoroid radiation emission, evaporation, melting, sputtering and kinetic energy of the ablated mass. Low entry velocities and grazing incidence angles favor micrometeoroid survival. Among those that survive, our model distinguishes (1) micrometeoroids who reach the melting temperature and for which melting is the most effective mass loss mechanism, and (2) micrometeoroids for which ablation due to evaporation causes most of the the mass loss. Melting is the most effective cooling mechanism. Sputtering-induced mass loss is negligible.
The nova T Pyx was observed with high resolution spectroscopy (R ~ 65000) spectroscopy, beginning 1 day after discovery of the outburst and continuing through the last visibility of the star at the end of May 2011. The interstellar absorption lines o
f Na I, Ca II, CH, CH$^+$, and archival H I 21 cm emission line observations have been used to determine a kinematic distance. Interstellar diffuse absorption features have been used to determine the extinction independent of previous assumptions. Sample Fe-peak line profiles show the optical depth and radial velocity evolution of the discrete components. We propose a distance to T Pyx $geq$4.5kpc, with a strict lower limit of 3.5 kpc (the previously accepted distance). We derive an extinction, E(B-V)$approx0.5pm$0.1, that is higher than previous estimates. The first observation, Apr. 15, displayed He I, He II, C III, and N III emission lines and a maximum velocity on P Cyg profiles of the Balmer and He I lines of $approx$2500 km s$^{-1}$ characteristic of the fireball stage. These ions were undetectable in the second spectrum, Apr. 23, and we use the recombination time to estimate the mass of the ejecta, $10^{-5}f$M$_odot$ for a filling factor $f$. Numerous absorption line systems were detected on the Balmer, Fe-peak, Ca II, and Na I lines, mirrored in broader emission line components, that showed an accelerated displacement in velocity. We also show that the time sequence of these absorptions, which are common to all lines and arise only in the ejecta, can be described by recombination front moving outward in the expanding gas without either a stellar wind or circumstellar collisions.
On 2010 Mar 10, V407 Cyg was discovered in outburst, eventually reaching V< 8 and detected by Fermi. Using medium and high resolution ground-based optical spectra, visual and Swift UV photometry, and Swift X-ray spectrophotometry, we describe the beh
avior of the high-velocity profile evolution for this nova during its first three months. The peak of the X-ray emission occurred at about day 40 with a broad maximum and decline after day 50. The main changes in the optical spectrum began at around that time. The He II 4686A line first appeared between days 7 and 14 and initially displayed a broad, symmetric profile that is characteristic of all species before day 60. Low-excitation lines remained comparatively narrow, with v(rad,max) of order 200-400 km/s. They were systematically more symmetric than lines such as [Ca V], [Fe VII], [Fe X], and He II, all of which showed a sequence of profile changes going from symmetric to a blue wing similar to that of the low ionization species but with a red wing extended to as high as 600 km/s . The Na I D doublet developed a broad component with similar velocity width to the other low-ionization species. The O VI Raman features were not detected. We interpret these variations as aspherical expansion of the ejecta within the Mira wind. The blue side is from the shock penetrating into the wind while the red wing is from the low-density periphery. The maximum radial velocities obey power laws, v(rad,max) t^{-n} with n ~ 1/3 for red wing and ~0.8 for the blue. (truncated)
