V4046 Sgr: Touchstone to Investigate Spectral Type Discrepancies for Pre-main Sequence Stars

TL;DR

This study uses near-infrared spectroscopy of the V4046 Sgr binary system to investigate discrepancies in spectral type determinations between optical and IR methods, highlighting implications for pre-main sequence star characterization.

Contribution

It provides a detailed case study demonstrating the differences in spectral type classification from optical versus near-IR data for a well-characterized pre-MS binary system.

Findings

Near-IR spectral type is M0-M1, later than optical K5+K7.

Optical spectral type aligns better with theoretical models.

Results caution against sole reliance on near-IR spectroscopy for stellar properties.

Abstract

Determinations of the fundamental properties (e.g., masses and ages) of late-type, pre-main sequence (pre-MS) stars are complicated by the potential for significant discrepancies between the spectral types of such stars as ascertained via optical vs. near-infrared observations. To address this problem, we have obtained near-IR spectroscopy of the nearby, close binary T Tauri system V4046 Sgr AB with the NASA Infrared Telescope Facility (IRTF) SPEX spectrometer. The V4046 Sgr close binary (and circumbinary disk) system provides an important test case for spectral type determination thanks to the stringent observational constraints on its component stellar masses (i.e., ~0.9 Msun each) as well as on its age (12-21 Myr) and distance (73 pc). Analysis of the IRTF data indicates that the composite near-IR spectral type for V4046 Sgr AB lies in the range M0-M1, i.e., significantly later than the K5+K7 composite type previously determined from optical spectroscopy. However, the K5+K7 composite type is in better agreement with theoretical pre-MS evolutionary tracks, given the well-determined properties of V4046 Sgr AB. These results serve as a cautionary tale for studies that rely on near-infrared spectroscopy as a primary means to infer the ages and masses of pre-MS stars.