Anthony L. Piro¹ and Ehud Nakar²

¹Theoretical Astrophysics, California Institute of Technology, 1200 E California Boulevard, M/C 350-17, Pasadena, CA 91125, USA
²Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

Ongoing transient surveys are presenting an unprecedented account of the rising light curves of Type Ia supernovae (SNe Ia). This early emission probes the shallowest layers of the exploding white dwarf (WD), which can provide constraints on the progenitor star and the properties of the explosive burning. We use semianalytic models of radioactively powered rising light curves to analyze these observations. As we have summarized in previous work, the main limiting factor in determining the surface distribution of ⁵⁶Ni is the lack of an unambiguously identified time of explosion, as would be provided by detection of shock breakout or shock-heated cooling. Without this the SN may in principle exhibit a “dark phase” for a few hours to days, where the only emission is from shock-heated cooling that is too dim to be detected. We show that by assuming a theoretically motivated time-dependent velocity evolution, the explosion time can be better constrained, albeit with potential systematic uncertainties. This technique is used to infer the surface ⁵⁶Ni distributions of three recent SNe Ia that were caught especially early in their rise. In all three we find fairly similar ⁵⁶Ni distributions. Observations of SN 2011fe and SN 2012cg probe shallower depths than SN 2009ig, and in these two cases ⁵⁶Ni is present merely ~10^-2 M_☉ from the WDs’ surfaces. The uncertainty in this result is up to an order of magnitude given the difficulty of precisely constraining the explosion time. We also use our conclusions about the explosion times to reassess radius constraints for the progenitor of SN 2011fe, as well as discuss the roughly t² power law that is inferred for many observed rising light curves.

Reference
Piro AL and Nakar E (2014) Constraints on Shallow 56Ni from the Early Light Curves of Type Ia Supernovae. The Astrophysical Journal 784:85.
[doi:10.1088/0004-637X/784/1/85]

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Bin Yang^1,2, Jacqueline Keane¹, Karen Meech^1,3, Tobias Owen³ and Richard Wainscoat³

¹NASA Astrobiology Institute, University of Hawaii, Honolulu, HI 96822, USA
²European Southern Observatory, Santiago, Chile
³Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA

The dynamically new comet C/2011 L4 (Pan-STARRS) is one of the brightest comets observed since the great comet C/1995 O1 (Hale-Bopp). Here, we present our multi-wavelength observations of C/2011 L4 during its in-bound passage to the inner solar system. A strong absorption band of water ice at 2.0 μm was detected in the near-infrared spectra, obtained with the 8 m Gemini-North and 3 m Infrared Telescope Facility Telescopes. The companion 1.5 μm band of water ice, however, was not observed. Spectral modeling shows that the absence of the 1.5 μm feature can be explained by the presence of sub-micron-sized fine ice grains. No gas lines (i.e., CN, HCN, or CO) were observed pre-perihelion in either the optical or the submillimeter. We derived 3σ upper limits for the CN and CO production rates. The comet exhibited a very strong continuum in the optical and its slope seemed to become redder as the comet approached the Sun. Our observations suggest that C/2011 L4 is an unusually dust-rich comet with a dust-to-gas mass ratio >4.

Reference
Yang B, Keane J, Meech K, Owen T and Wainscoat R (2014) TMulti-wavelength Observations of Comet C/2011 L4 (Pan-STARRS). The Astrophysical Journal – Letters 784:L23.
[doi:10.1088/2041-8205/784/2/L23]

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Shoshana Z. Weider^a,d, Larry R. Nittler^a,d, Richard D. Starr^b,d, Timothy J. McCoy^c,d and Sean C. Solomon^a,c,d

^aDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
^bPhysics Department, The Catholic University of America, Washington, DC 20064, USA
^cDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
^dLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

We present measurements of Mercury’s surface composition from the analysis of MESSENGER X-Ray Spectrometer data acquired during 55 large solar flares, which each provide a statistically significant detection of Fe X-ray fluorescence. The Fe/Si data display a clear dependence on phase angle, for which the results are empirically corrected. Mercury’s surface has a low total abundance of Fe, with a mean Fe/Si ratio of ∼0.06 (equivalent to ∼1.5 wt % Fe). The absolute Fe/Si values are subject to a number of systematic uncertainties, including phase-angle correction and possible mineral mixing effects. Individual Fe/Si measurements have an intrinsic error of ∼10%. Observed Fe/Si values display small variations (significant at two standard deviations) from the planetary average value across large regions in Mercury’s southern hemisphere. Larger differences are observed between measured Fe/Si values from more spatially resolved footprints on volcanic smooth plains deposits in the northern hemisphere and from those in surrounding terrains. Fe is most likely contained as a minor component in sulfide phases (e.g., troilite, niningerite, daubréelite) and as Fe metal, rather than within mafic silicates. Variations in surface reflectance (i.e., differences in albedo and spectral slope) across Mercury are unlikely to be caused by variations in the abundance of Fe.

Reference
Weider SZ, Nittler LR, Starr RD, McCoy TJ and Solomon SC (in press) Variations in the abundance of iron on Mercury’s surface from MESSENGER X-Ray Spectrometer observations. Icarus
[doi:10.1016/j.icarus.2014.03.002]
Copyright Elsevier

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Mainzer¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Only a very small fraction of the asteroid population at size scales comparable to the object that exploded over Chelyabinsk, Russia has been discovered to date, and physical properties are poorly characterized. We present previously unreported detections of 105 close approaching near-Earth objects (NEOs) by the Wide-field Infrared Survey Explorer (WISE) mission’s NEOWISE project. These infrared observations constrain physical properties such as diameter and albedo for these objects, many of which are found to be smaller than 100 m. Because these objects are intrinsically faint, they were detected by WISE during very close approaches to the Earth, often at large apparent on-sky velocities. We observe a trend of increasing albedo with decreasing size, but as this sample of NEOs was discovered by visible light surveys, it is likely that selection biases against finding small, dark NEOs influence this finding.

Reference
Mainzer et al. (2014) The Population of Tiny Near-Earth Objects Observed by NEOWISE. The Astrophysical Journal 784:110.
[doi:10.1088/0004-637X/784/2/110]

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