Context. The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and other stars are at the focus of intense contemporary research. Aims. Observations at submm/mm wavelengths are particularly useful to obtain information about the run of the temperature in the upper atmosphere of Sun-like stars. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to study the chromospheric emission of the α Centauri binary system in all six available frequency bands during Cycle 2 in 2014-2015. Methods. Since ALMA is an interferometer, the multi-telescope array is particularly suited for the observation of point sources. With its large collecting area, the sensitivity is high enough to allow the observation of nearby main-sequence stars at submm/mm wavelengths for the first time. The comparison of the observed spectral energy distributions with theoretical model computations provides the chromospheric structure in terms of temperature and density above the stellar photosphere and the quantitative understanding of the primary emission processes. Results. Both stars were detected and resolved at all ALMA frequencies. For both α Cen A and B, the existence and location of the temperature minima, firstly detected from space with Herschel, are well reproduced by the theoretical models of this paper. For α Cen B, the temperature minimum is deeper than for A and occurs at a lower height in the atmosphere, but for both stars, T min /T eff is consistently lower than what is derived from optical and UV data. In addition, and as a completely different matter, a third point source was detected in Band 8 (405 GHz, 740 µm) in 2015. With only one epoch and only one detection, we are left with little information regarding that object's nature, but conjecture that it might be a distant solar system object. Conclusions. The submm/mm emission of the α Cen stars is indeed very well reproduced by modified chromospheric models of the Quiet Sun. This most likely means that the non-radiative heating mechanisms of the upper atmosphere that are at work in the Sun are operating also in other solar-type stars.
We present the geochemical and isotopic characteristics of Pleistocene to Holocene olivine-bearing mafic rocks from the Sierra de Chichinautzin (SCN) volcanic field, located in the central part of the Mexican Volcanic Belt (MVB). Some have geochemical characteristics of primary magmas, and their MORB-normalized patterns are similar to those displayed by extension-related mafic rocks. The SCN primary magmas show a limited range in 87 Sr/ 86 Sr (0.70348-0.70397; average (n = 7) = 0.70370 ± 0.00019), and 143 Nd/ 144 Nd (0.51279-0.51294; average (n = 6) = 0.51288 ± 0.00006).~New mineral and whole-rock chemical data allow estimation of eruption temperatures and assure the near-primary nature of the studied samples. Equilibrium temperatures for the primary magmas were in the range 1070-1150°C. Trace-element concentration data for near-primary magmas from the SCN are used to develop a partial melting inversion model, the first for any area of the MVB. The source composition calculated by this inversion method shows an enrichment in highly incompatible elements (e.g., [C La /C Yb ] n ~2.2), where [] n represents mande-normalized values), without a decou pling between LILE, REE, and HFSE. This relationship supports a rift-related origin for SCN volcanism. The calculated REE composition of the mantle source is comparable to that observed in mande xenoliths of central Mexico. Concentration ratios of incompatible elements have also been used to infer 7-16% as the approximate range in degree of partial melting of the mantle. These results reinforce the hypothesis that SCN mafic magmatism reflects partial melting of an enriched lithospheric mantle in an extensional tectonic setting, precluding participation of the subducted Cocos plate in the genesis of these magmas.
Modeling the submillimeter to centimeter emission of stars is challenging due to a lack of sensitive observations at these long wavelengths. We launched an ongoing campaign to obtain new observations entitled Measuring the Emission of Stellar Atmospheres at Submillimeter/millimeter wavelengths (MESAS). Here we present ALMA, GBT, and VLA observations of Sirius A, the closest main-sequence A-type star, that span from 1.4 to 9.0 millimeters. These observations complement our previous millimeter data on Sirius A and are entirely consistent with the PHOENIX stellar atmosphere models constructed to explain them. We note that accurate models of long wavelength emission from stars are essential not only to understand fundamental stellar processes, but also to determine the presence of dusty debris in spatially unresolved observations of circumstellar disks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.