Physical parameters of late M-type members of Chamaeleon I and TW Hydrae Association: dust settling, age dispersion and activity

Abstract

Although mid-to-late type M dwarfs are the most common stars in our stellar neighborhood, our knowledge of these objects is still limited. Open questions include the evolution of their angular momentum, internal structures, dust settling in their atmospheres, age dispersion within populations. In addition, at young ages, late-type Ms have masses below the hydrogen burning limit and therefore are key objects in the debate on the brown dwarf mechanism of formation. In this work we determine and study in detail the physical parameters of two samples of young, late M-type sources belonging to either the Chamaeleon I Dark Cloud or the TW Hydrae Association and compare them with the results obtained in the literature for other young clusters and also for older, field, dwarfs. We used multi-wavelength photometry to construct and analyze SEDs to determine general properties of the photosphere and disk presence. We also used low resolution optical and near-infrared spectroscopy to study activity, accretion, gravity and effective temperature sensitive indicators. We propose a VO-based spectral index that is both temperature and age sensitive. We derived physical parameters using independent techniques confirming the already common feature/problem of the age/luminosity spread. In particular, we highlight two brown dwarfs showing very similar temperatures but clearly different surface gravity (explained invoking extreme early accretion). We also show how, despite large improvement in the dust treatment in theoretical models, there is still room for further progress in the simultaneous reproduction of the optical and near-infrared features of these cold young objects.

New low-mass members of Chamaeleon I and $ε$ Cha

“…On the other hand, they also report a trend towards determination of higher temperatures when using only optical data versus including the near-infrared spectra in the fit. In the latter case, the best-fit parameters are more in line with those from the SED fitting, which is probably not surprising given that the SEDs in Bayo et al (2017), as well as in this work, contain both optical and near-infrared photometry.…”

“…the extinction close to zero may be expected. A similar trend is present in the results of Bayo et al (2017), when comparing the T eff and A V derived from fitting the BT-Settl models to the optical spectra, and the SED fitting. On the other hand, they also report a trend towards determination of higher temperatures when using only optical data versus including the near-infrared spectra in the fit.…”

“…However, as previously noted by e.g. Bayo et al (2017), the SED fitting procedure is largely insensitive to logg, resulting in flat probability density distribu- Fig. 5.…”

First Millimeter Detection of the Disk around a Young, Isolated, Planetary-mass Object

“…2, are consistent with each other since we recalculated the dust masses using the same absorption coefficient (to rescale the values in Testi et al 2016). In the case where T dust scales with the bolometric luminosity of the central object, the same caution was taken in the use of consistent opacities, and we adopted the stellar parameters from Manara et al (2014Manara et al ( , 2016 and Manara et al (2017), for the Pascucci et al (2016) sample (Cha I objects, as OTS44), where those values are generally in agreement with Luhman (2004) or more recently Bayo et al (2017).…”

A new take on the low-mass brown dwarf companions on wide orbits in Upper-Scorpius

“…We assumed a solar metallicity M/H = 0.0, in broad agreement with the values reported in Sco-Cen (Bubar et al 2011). These grids have been extensively utilized in previous studies of young BDs (Manjavacas et al 2014;Bonnefoy et al 2014;Delorme et al 2017b;Bayo et al 2017), but never -1913 B (black) to those of reference objects scaled to the companion's distance (green). The companion is 2.0 to 3.9 times more luminous than objects having the same spectral type.…”