The Young Solar Analogs Project. I. Spectroscopic and Photometric Methods and Multi-Year Timescale Spectroscopic Results

Abstract

This is the first in a series of papers presenting methods and results from the Young Solar Analogs Project, which began in 2007. This project monitors both spectroscopically and photometrically a set of 31 young (300-1500 Myr) solar-type stars with the goal of gaining insight into the space environment of the Earth during the period when life first appeared. From our spectroscopic observations we derive the Mount Wilson S chromospheric activity index (S MW ), and describe the method we use to transform our instrumental indices to S MW without the need for a color term. We introduce three photospheric indices based on strong absorption features in the blue-violet spectrum-the G-band, the Ca I resonance line, and the Hydrogen-γ line-with the expectation that these indices might prove to be useful in detecting variations in the surface temperatures of active solar-type stars. We also describe our photometric program, and in particular our "Superstar technique" for differential photometry which, instead of relying on a handful of comparison stars, uses the photon flux in the entire star field in the CCD image to derive the program star magnitude. This enables photometric errors on the order of 0.005-0.007 magnitude. We present time series plots of our spectroscopic data for all four indices, and carry out extensive statistical tests on those time series demonstrating the reality of variations on timescales of years in all four indices. We also statistically test for and discover correlations and anti-correlations between the four indices. We discuss the physical basis of those correlations. As it turns out, the "photospheric" indices appear to be most strongly affected by emission in the Paschen continuum. We thus anticipate that these indices may prove to be useful proxies for monitoring emission in the ultraviolet Balmer continuum. Future papers in this series will discuss variability of the program stars on medium (days-months) and short (minutes to hours) timescales.

Relationship between Ca and Hα chromospheric emission in F-G-K stars: Indication of stellar filaments?

“…The spectra were retrieved from the European Southern Observatory (ESO) archive. These stars have been observed and studied in various surveys and in particular by Sousa et al (2008), Ramírez et al (2014), Marsden et al (2014), Datson et al (2014), Borgniet et al (2017), Lagrange et al (2013), and Gray et al (2015). We have already studied most of these stars in Meunier et al (2017) to estimate the amplitude of their convective blueshift and its dependence on the activity level.…”

Variability in stellar granulation and convective blueshift with spectral type and magnetic activity

“…Type of survey Number of stars 1 Sousa et al (2008) F-G-K stars, old, not very active, exoplanet survey 294 2 Ramírez et al (2014) Solar twins, old and young stars 64 3 Marsden et al (2014) Solar-type stars, old and young (Bcool survey) 31 4 Datson et al (2014) Solar twins 5 5 Borgniet et al (2017) F stars, old stars, exoplanet survey 6 6 Lagrange et al (2013) Young stars, exoplanet survey 8 7 Gray et al (2015) Young solar twins 4…”

“…Our sample includes 360 stars with spectral types from F7 to K4 (spectral types and B−V values were retrieved from the Simbad database at the CDS 1 ), observed with HARPS by several groups: Sousa et al (2008), Ramírez et al (2014), Marsden et al (2014), Datson et al (2014), Borgniet et al (2017), Lagrange et al (2013), and Gray et al (2015). The spectra are available in the ESO archives, and we selected only spectra with an average signal-to-noise ratio above 100, leading to a total of 19 510 spectra (i.e., 54 spectra on average per star, a median value of 25, and a number of spectra per star from 2 to 1230).…”

Relationship between Ca and H$α$ chromospheric emission in F-G-K stars: indication of stellar filaments?

“…The spectra were retrieved from the European Southern Observatory (ESO) archive. These stars have been observed and studied in various surveys and in particular by Sousa et al (2008), Ramírez et al (2014), Marsden et al (2014), Datson et al (2014), Borgniet et al (2017), Lagrange et al (2013), andGray et al (2015). We have already studied most of these stars in Meunier et al (2017) to estimate the amplitude of their convective blueshift and its dependence on the activity level.…”