EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity)

Abstract

Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm–cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars. Aims. We study the spectral energy distribution (Sobs(ν)) of main-sequence stars based on archival observations in the mm–cm range with the aim to study their atmospheric stratification as a function of stellar type. Methods. The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet to cm range as compiled from various catalogues and observatory archives. We compared the resultant Sobs(ν) of each star with a photospheric emission model (Smod(ν)) calculated with the PHOENIX code. The departures of Sobs(ν) from Smod(ν) were quantified in terms of a spectral flux excess parameter (ΔS∕Smod) and studied as a function of stellar type. Results. The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sun-as-a-star as reference. The stars with Teff = 3000–7000 K (F–M type) showed a systematically higher Sobs(ν) than Smod(ν) in the mm–cm range. Their ΔS∕Smod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the Teff = 3000–7000 K range, although the single fully convective star (Teff ~ 3000 K) in the sample deviates from this trend. Meanwhile, Sobs(ν) of the A-type stars agrees with Smod(ν) within errors. Conclusions. The systematically high ΔS∕Smod in F–M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona in these stars, like for the Sun. The mm–cm ΔS∕Smod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm–cm range.

First Millimeter Flares Detected from ϵ Eridani with the Atacama Large Millimeter/submillimeter Array

“…An inner warm component several astronomical units from the star has been inferred from infrared observations (e.g., Backman et al 2009;Greaves et al 2014) (2015) also noted a flux excess at 7 mm with ATCA, which implied an increasing effective temperature with increasing wavelength, characteristic of stellar chromospheric emission as has been observed previously from α Cen A and B (Liseau et al 2015(Liseau et al , 2016. Mohan et al (2021) fit a PHOENIX stellar atmosphere model to the available long wavelength data with a temperature of 5100 K.…”

EMISSA (Exploring millimetre indicators of solar-stellar activity)

“…The mm stellar sample compiled by Mohan et al (2021) was used for the study. The compiled T B (ν) data come from observations done over years, providing chromospheric thermal profiles averaged over activity cycle timescales.…”

“…White et al 2017White et al , 2020Trigilio et al 2018). In the previous paper in this series (Paper I; Mohan et al 2021), the mm-T B (ν) spectrum was derived for main-sequence stars using archival ALMA data. The sample was made by systematically avoiding stars with potential flux contamination from known debris disks or unresolved companion stars.…”

Exoplanet Radio Transits as a Probe for Exoplanetary Magnetic Fields—Time-dependent MHD Simulations

“…Previous studies already indicated the possibility of detecting the signature of exoplanet transit around the host star using upcoming SKA in the low frequency range (Pope et al 2019) and ALMA in the high frequency range (Selhorst et al 2013;Selhorst & Barbosa 2020). However, radio flux from the planet-hosting star should be sufficiently strong to be observable; one may use few known stars with observable radio flux for that purpose (see Wendker 1995;Slee et al 2003;Lecavelier Des Etangs et al 2011;Villadsen et al 2014;Fichtinger et al 2017;Mohan et al 2021). Many of these previous studies placed an upper limit on the flux density and mass-loss rates on the wind of host stars.…”