A Revised Age for Upper Scorpius and the Star Formation History Among the F-Type Members of the Scorpius-Centaurus Ob Association

Abstract

We present an analysis of the ages and star-formation history of the F-type stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus (Sco-Cen), the nearest OB association. Our parent sample is the kinematically-selected Hipparcos sample of de Zeeuw et al. (1999), restricted to the 138 F-type members. We have obtained classification-resolution optical spectra and have also determined the spectroscopic accretion disk fraction. With Hipparcos and 2MASS photometry, we estimate the reddening and extinction for each star and place the candidate members on a theoretical H-R diagram. For each subgroup we construct empirical isochrones and compare to published evolutionary tracks. We find that 1) our empirical isochrones are consistent with the previously published age-rank of the Sco-Cen subgroups, 2) subgroups LCC and UCL appear to reach the main sequence turn-on at spectral types ∼F4 and ∼F2, respectively. An analysis of the A-type stars shows US reaching the main sequence at about spectral type ∼A3.3) The median ages for the pre-main sequence members of UCL and LCC are 16 Myr and 17 Myr, respectively, in agreement with previous studies, however we find that 4) Upper Sco is much older than previously thought. The luminosities of the F-type stars in US are typically a factor of ∼2.5 less luminous than predicted for a 5 Myr old population for four sets of evolutionary tracks. We re-examine the evolutionary state and isochronal ages for the B-, A-, and G-type Upper Sco members, as well as the evolved M supergiant Antares, and estimate a revised mean age for Upper Sco of 11±1±2 Myr (statistical, systematic). Using radial velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic expansion age for Upper Sco of >10.5 Myr (99% confidence). However, the data are statistically consistent with no expansion. We reevaluate the inferred masses for the known substellar companions in Upper Sco using the revised age and find the inferred masses are typically ∼20-70% higher than the original estimates which had assumed a much younger age; specifically, we estimate the mass of 1RXS J1609-2105b to be 14 +2 −3 M Jup , suggesting that it is a brown dwarf rather than a planet. Finally, we find the fraction of F-type stars exhibiting Hα emission and/or a K-band excess consistent with accretion to be 0/17 (< 19%; 95% C.L.) in US at ∼11 Myr, while UCL has 1/41 (2 +5 −1 %; 68% C.L.) accretors and LCC has 1/50 (2 +4 −1 %; 68% C.L.) accretors at ∼16 Myr and ∼17 Myr, respectively.

Keplerobservations of A–F pre-main-sequence stars in Upper Scorpius: discovery of six new δ Scuti and one γ Doradus stars

“…As we discussed in Sect. 4, there is some disagreement between our own results and those of Pecaut, Mamajek, & Bubar (2012). However, we note that the use of Pecaut, Table 2. combination of empirical isochrones and the models of Dotter et al (2008) and found an age for USco of 11±1 Myr.…”

“…The only relevant exceptions are EPIC 204399980 (HIP 79476) and 204494885 (HIP 80130). For the first star we find an effective temperature about 500 K larger than that reported by Pecaut, Mamajek, & Bubar (2012), who in turn refer to Vieira et al (2003). However, on the basis of high-resolution spectroscopy, Carmona et al (2010) discussed the spectral type of this star, concluding that it cannot be later than A5Ve and estimated a T spec eff =8200 K, almost identical to our value.…”

“…Synthetic spectra were generated in three steps: i) we computed LTE atmospheric models using the ATLAS9 code; ii) the stellar spectra were synthesized using SYNTHE; iii) the spectra were convolved for the instrumental and rotational broadening 3 . We used as initial guesses for temperatures the values derived by Pecaut, Mamajek, & Bubar (2012), that we report for the sake of clarity in Table 2. In Fig.…”

“…This is the case, for example, of NGC 2264, a well-studied cluster where 9 PMS pulsators have been observed from space (Zwintz et al 2014). The Upper Scorpius association (USco) is perhaps of even greater interest due to the large number of known A-F members (de Zeeuw et al 1999;Pecaut, Mamajek, & Bubar 2012) and the availability of accurate individual distances based on Hipparcos parallaxes. Moreover, stellar parameters have been estimated for most of the member stars.…”

A rocky composition for an Earth-sized exoplanet

“…We conclude that our detection of Kepler-78 is not strongly sensitive to spot model assumptions or to individual RV measurements. 44 . Additional iron and calcium lines are labeled.…”