Survival of carbon grains in shocks

Supernova shocks play a significant part in the life of an interstellar grain. In a typical 10 to the 9th power year lifetime, a grain will be hit by an average of 10 shocks of 100 km s(sup -1) or greater velocity, and even more shocks of lower velocity. Evaluation of the results of this frequent shock processing is complicated by a number of uncertainties, but seems to give about 10 percent destruction of silicate grains and about half that for graphite grains. Because of the frequency of shocking, the mineralogy and sizes of the grain population is predominately determined by shock processing effects, and not by the initial grain nucleation and growth environment. One consequence of the significant role played by interstellar shocks is that a certain fraction (up to 5 percent) of the carbon should be transformed into the diamond phase. Diamond transformation is observed in the laboratory at threshold shock pressures easily obtainable in grain-grain collisions in supernova shocks. Yields for transforming graphite, amorphous carbon, glassy carbon, and other nearly pure carbon solids into diamond are quite high. Impurities up to at least the 10 percent level (for oxygen) are tolerated in the process. The typical size diamond expected from shock transformation agrees well with the observed sizes in the Lewis et al. findings in meteoritic material. Isotropic anomalies already contained in the grain are likely to be retained through the conversion process, while others may be implanted by the shock if the grain is close to the supernova. The meteoritic diamonds are likely to be the results of transformation of carbon grains in grain-grain collisions in supernova shock waves

The origin of micrograins

Using ultraviolet and infrared techniques, researchers investigated the origins of the tiny (approx. 10A) grains whose presence in the interstellar medium (ISM) is inferred from near-infrared photometry (Sellgren, Werner, and Dinerstein 1983; Sellgren 1984). The authors consider two possibilities: (1) that the grains are formed by condensation in stellar atmospheres; or (2) that they are formed by fragmentation of larger grains in interstellar shocks. They searched for evidence of very small grains in circumstellar environments by analyzing ultraviolet extinction curves in binaries containing hot companions, and by searching for the 3.3-micron emission feature in similar systems. The ultraviolet extinction curve analysis could be applied only to oxygen-rich systems, where small carbonaceous grains would not be expected, so these results provide only indirect information. Researchers find a deficiency of grains smaller than 800A in oxygen-rich systems, consistent with theoretical models of grain condensation which suggest that grains grow to large sizes before injection into the interstellar medium. More direct information on carbonaceous micrograins was obtained from the search for the 3.3-micron feature in carbon-rich binaries with hot companions, whose ultraviolet flux should excite the tiny grains to emit in the infrared. No 3.3-micron feature was found, suggesting that the micrograins are absent in these systems. In addition to the negative search for micrograins in circumstellar environments, researchers have also studied the possible association of these grains with shocks in the diffuse interstellar medium. Using Infrared Astronomy Satellite (IRAS) colors as indicators of the presence or absence of the small grains (e.g., Ryter, Puget, and Perault 1987 and references cited therein), researchers systematically searched for them in regions (reflection nebulae) expected to have sufficient ultraviolet flux to make them glow in the infrared. They found that the distribution is not uniform. The researchers propose that production of micrograins by fragmentation of larger grains in shocks could explain this uneven distribution

Ancient giants: on the farthest galaxy at z=8.6

The observational frontiers for the detection of high-redshift galaxies have recently been pushed to unimaginable distances with the record-holding Lyman Alpha Emitter (LAE) UDFy-38135539 discovered at redshift z=8.6. However, the physical nature and the implications of this discovery have yet to be assessed. By selecting galaxies with observed luminosities similar to UDFy-38135539 in state-of-the-art cosmological simulations tuned to reproduce the large scale properties of LAEs, we bracket the physical nature of UDFy-38135539: it has a star formation rate ~ 2.7-3.7 solar masses/yr, it contains ~ 10^{8.3-8.7} solar mass of stars 50-80 Myr old, with stellar metallicity ~ 0.03-0.12 of the solar value. For any of the simulated galaxies to be visible as a LAE in the observed range, the intergalactic neutral hydrogen fraction at z=8.6 must be <= 0.2 and extra ionizing radiation from sources clustered around UDFy-38135539 is necessary. Finally, we predict that there is a 70% (15%) probability of detecting at least 1 such source from JWST (HST/WFC3) observations in a physical radius ~ 0.4 Mpc around UDFy-38135539.Comment: Accepted to MNRAS letter

Diffuse band profiles in the Rho Ophiuchi cloud

High-resolution, high signal-to-noise ratio line profiles are presented for the 5780 and 5797 A diffuse interstellar bands toward six stars in the Rho Oph dark cloud. Target stars were chosen to exhibit a wide range of interstellar grain properties, as measured by grain polarization and far-UV extinction. The extreme case of the heavily reddened star HD 147889 is included; this star has one of the highest known lambdamax values, indicative of unusually large grains. Despite the differences in the grain properties, the line profiles and central wavelengths for the 5780 A band were found to be essentially identical for all lines of sight. This finding is in contradiction to the results of the embedded cavity grain model for diffuse bands, which predicts changes in both profile and central wavelength with grain size and impurity concentration. Results therefore support a molecular origin for the diffuse bands

Diffuse band profiles in the spectrum of HD 29647: Evidence for a molecular origin?

High signal-to-noise ratio spectra have been obtained of the diffuse interstellar bands at 5780 and 5797 Å in the spectrum of HD 29647, a heavily reddened star within or behind a portion of the Taurus dark cloud complex. The observations were made using the coudé spectrograph on the Canada-France-Hawaii Telescope. The Reticon detector combined with the coudé spectrograph and excellent observing conditions allowed S/N ratios as high as 200 for this star, which was V=8.37 and E(B–V)=1.03. In two separate exposures both bands were found to be narrower and weaker than normal values for stars of similar reddening, and the profiles appear to deviate from those normally seen as well. Theories of band formation due to absorption centers in solid grains require bandwidths much greater than observed in HD 29647 and predict profile variations with grain size that are quite different from what is seen. Therefore we suggest that these observations argue for a molecular origin for the diffuse bands. The observed profiles may be explained as due to unusual rotational excitation in molecules

Diffuse band profiles in the Rho Ophiuchi cloud

High-resolution, high signal-to-noise ratio line profiles are presented for the 5780 and 5797 A diffuse interstellar bands toward six stars in the Rho Oph dark cloud. Target stars were chosen to exhibit a wide range of interstellar grain properties, as measured by grain polarization and far-UV extinction. The extreme case of the heavily reddened star HD 147889 is included; this star has one of the highest known lambdamax values, indicative of unusually large grains. Despite the differences in the grain properties, the line profiles and central wavelengths for the 5780 A band were found to be essentially identical for all lines of sight. This finding is in contradiction to the results of the embedded cavity grain model for diffuse bands, which predicts changes in both profile and central wavelength with grain size and impurity concentration. Results therefore support a molecular origin for the diffuse bands