Dynamical Masses of Young Stars. II. Young Taurus Binaries Hubble 4, FF Tau, and HP Tau/G3

Abstract

One of the most effective ways to test stellar evolutionary models is to measure dynamical masses for binary systems at a range of temperatures. In this paper, we present orbits of three young K+M binary systems in Taurus (Hubble 4, FF Tau, and HP Tau/G3) with VLBI parallaxes. We obtained precision astrometry with Keck-II/NIRC2, optical photometry with HST/WFC3, and low-resolution optical spectra with WIFeS on the ANU 2.3 m telescope. We fit orbital solutions and dynamical masses with uncertainties of 1-5% for the three binary systems. The spectrum, photometry, and mass for Hubble 4 are inconsistent with a binary system, suggesting that it may be a triple system where the primary component consists of two stars. For HP Tau/G3 and FF Tau, model masses derived from SED determined component temperatures and luminosities agree with the dynamical masses, with a small offset towards larger model masses. We find model ages for the primary components of these systems of ∼3 Myr, but find that the secondaries appear younger by a factor of two. These estimates also disagree with the age of the physically associated G-type star HP Tau/G2, which is older (∼5 Myr) according to the same models. This discrepancy is equivalent to a luminosity under-prediction of 0.1-0.2 dex, or a temperature over-prediction of 100-300 K, for K/M-type stars at a given model age. We interpret this as further evidence for a systematic error in pre-main sequence evolutionary tracks for convective stars. Our results reinforce that the ages of young populations determined from the locus of M-type members on the HR-diagram may require upward revision.

The Gaia-ESO survey: a lithium depletion boundary age for NGC 2232

“…Further indications of significant problems in the physics of low-mass PMS stars come from discrepancies between isochronal ages and the amount (and dispersion) of lithium that PMS stars deplete as they contract and their cores become hot enough to "burn" lithium (Jeffries et al 2017;Bouvier et al 2018). There are also direct and indirect indications that magnetically active stars, whether they are fast-rotating and young or members of close, tidally-locked binary systems, have larger radii than predicted by the most commonly used stellar models (Morales et al 2009;Torres 2013;Malo et al 2014b;Kraus et al 2015Kraus et al , 2017Rizzuto et al 2020). This has led to suggestions that rotation, magnetic fields and high surface coverage of starspots may significantly alter the evolutionary tracks and isochrones in young clusters (Feiden & Chaboyer 2013;Jackson & Jeffries 2014a;Somers & Pinsonneault 2015;MacDonald & Mullan 2017).…”

Radial Velocity Monitoring of the Young Star Hubble 4: Disentangling Starspot Lifetimes from Orbital Motion

“…The HST imaging allowed us to measure the optical flux ratio of the two members of the binary and that work is presented in detail in the Appendix B (where it is also compared to the recent analysis of Rizzuto et al 2020). The HST analysis shows that the optical flux ratio F 2 /F 1 is ∼ 0.8, demonstrating that both stars contribute substantially to the observed optical light.…”

“…However, the RV values that we measure in Hubble 4 have two sources: the spot-induced signals we wish to study and also signals resulting from the binary nature of the Hubble 4 system. The binary properties of Hubble 4 are well established (Galli et al 2018;Rizzuto et al 2020), therefore, for the purpose of this study, the binary signal needs to be subtracted out so that the spot-induced signals can be isolated and studied. This is done in a two step process.…”

“…Thus, we conclude the offsets in the RVs and bisector spans are real and potentially reflect the motion resulting from the long period binary. Galli et al (2018) used their own VLBA observations combined with Keck/NIRC2 data (later published by Rizzuto et al 2020) to determine the orbital motion of the binary and compute the orbital parameters of the system, summarized in Table 1. We use these parameters to fit the orbital contribution to the RV variations of Hubble 4.…”

“…It is a highly magnetically active K7 star, with a mean surface field strength of 2.5 kG (Johns-Krull et al 2004), and it was found to have large amplitude RV variations attributed to a spot (Mahmud et al 2011). In much of the relevant literature, Hubble 4 is described as a single object; however, recent studies have shown it to be a close binary system (Kraus et al 2011;Galli et al 2018;Rizzuto et al 2020). Hints of this binarity were reported earlier in a study of the radio emission from Hubble 4: Skinner (1993) found the emission was extended in such a way that might imply the presence of a companion.…”