Browsing by Author "Mathieu, Robert D."

We show high-resolution spectra of the eclipsing brown dwarf binary 2MASS J05352184 - 0546085 taken at the two opposite radial velocity maxima. Comparisons of the TiO bands to model and template spectra are fully consistent with the temperatures previously derived for this system. In particular, the reversal of temperatures with mass - in which the higher mass primary is cooler than its companion - is confirmed. We measure the projected rotation velocities of the components; the primary is rotating at least twice as rapidly as the secondary. At the two radial velocity maxima, H alpha emission lines of both components stick out to either sides of the H alpha central wavelength, which is dominated by nebula emission. This enables us to model the individual H alpha lines of the primary and the secondary. We find that the H alpha emission from the primary is at least 7 times stronger than the emission from the secondary. We conclude that the temperature reversal is very likely due to strong magnetic fields inhibiting convection on the primary.

We developed an experimental setup to test the hypothesis that accretionary rims around chondrules formed in the solar nebula by accretion of dust on the surfaces of hot chondrules. Our experimental method allows us to form dust rims around chondrule analogs while levitated in an inert-gas flow. We used micrometer-sized powdered San Carlos olivine to accrete individual dust particles onto the chondrule analogs at room temperature (20 degrees C) and at 1100 degrees C. The resulting dust rims were analyzed by means of two different techniques: non-destructivemicro computer tomography, and scanning electron microscopy. Both methods give very similar results for the dust rimstructure and a mean dust rim porosity of 60% for the hot coated samples, demonstrating that both methods are equally well suited for sample analysis. The chondrule analog's bulk composition has no measurable impact on the accretion efficiency of the dust. We measured the chemical composition of chondrule analog and dust rim to check whether elemental exchange between the two components occurred. Such a reaction zone was not found; thus, we can experimentally confirm the sharp border between chondrules and dust rims described in the literature. We adopted a simple model to derive the degree of post-accretionary compaction for different carbonaceous chondrites. Moreover, we measured the rim porosity of a fragment of Murchison meteorite, analyzed it with micro-CT and found rim porosities with this technique that are comparable to those described in the literature. (C) 2012 Elsevier Ltd. All rights reserved.

The sodium solubility in silicate melts in the CaO-MgO-SiO2 (CMS) system at 1400 C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na-(g) evaporation from a Na2O-xSiO(2) melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa(2)O((sample)) = aNa(2)O((source)). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na2O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO2 < 100; in wt%), we found that Na2O solubility is conveniently modeled as a linear function of the optical basicity (A) calculated on a Na-free basis melt composition. In our experiments, gamma Na2O(sample) ranges from 7 x 10(-7) to 5 x 10(-6), indicating a strongly non-ideal behavior of Na2O solubility in the studied CMS melts (gamma Na2O(sample) << 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na2O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na2O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q(4). (C) 2010 Elsevier Ltd. All rights reserved.

Sub-subgiants are stars that are. observed to be redder than normal main-sequence stars and fainter than normal subgiant (and giant) stars in an optical color-magnitude diagram (CMD). The red straggler stars, which lie redward of the red giant branch, may be related and are often grouped together with the sub-subgiants in the literature. These stars defy our standard theory of single-star evolution. and are important tests for binary evolution and stellar collision models. In total, we identify 65 sub-subgiants (SSG) and red stragglers (RS) in 16 open and globular star clusters from the literature; 50 of these, including 43 sub-subgiants, pass our strict membership selection criteria (though the remaining sources may also be cluster members). In addition to their unique location on the CMD, we find that at least 58% (25/43) of sub-subgiants in this sample are X-ray sources with typical 0.5-2.5 keV luminosities of order 10(30)-10(31) erg s(-1). Their X-ray luminosities and optical-to-X-ray flux ratios are similar to those of RS CVn active binaries. At least 65% (28/43) of the sub-subgiants in our sample are variables, 21 of which are known to be radial-velocity binaries. Typical variability periods are less than or similar to 15. days. At least 33% (14/43) of the sub-subgiants are Ha emitters. These observational demographics provide strong evidence that binarity is important for sub-subgiant formation. Finally, we find that the number of sub-subgiants per unit mass increases toward lower-mass clusters, such that the open clusters in our sample have the highest specific frequencies of sub-subgiants.