ABSTRACT Studies of T Tauri discs inform planet formation theory; observations of variability due to occultation by circumstellar dust are a useful probe of unresolved, planet-forming inner discs, especially around faint M dwarf stars. We report observations of 2M0632, an M dwarf member of the Carina young moving group that was observed by Transiting Exoplanet Survey Satellite over two 1-yr intervals. The combined light curve contains >300 dimming events, each lasting a few hours, and as deep as 40 per cent (0.55 magnitudes). These stochastic events are correlated with a distinct, stable 1.86-d periodic signal that could be stellar rotation. Concurrent ground-based, multiband photometry show reddening consistent with interstellar medium-like dust. The star’s excess emission in the infrared and emission lines in optical and infrared spectra reveal a T Tauri-like accretion disc around the star. We confirm membership of 2M0632 in the Carina group by a Bayesian analysis of its Galactic space motion and position. We combine stellar evolution models with Gaia photometry and constraints on Teff, luminosity, and the absence of detectable lithium in the photosphere to constrain the age of the group and 2M0632 to 40–60 Myr, consistent with earlier estimates. 2M0632 joins a handful of long-lived discs which challenge the canon that disc lifetimes are ≲10 Myr. All known examples surround M dwarfs, suggesting that lower X-ray/ultraviolet irradiation and slower photoevaporation by these stars can dramatically affect disc evolution. The multiplanet systems spawned by long-lived discs probably experienced significant orbital damping and migration into close-in, resonant orbits, and perhaps represented by the TRAPPIST-1 system.
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An ALMA Survey of M-dwarfs in the Beta Pictoris Moving Group with two new debris disc detections
Previous surveys in the far-infrared have found very few, if any, M-dwarf debris discs among their samples. It has been questioned whether M-dwarf discs are simply less common than earlier types, or whether the low detection rate derives from the wavelengths and sensitivities available to those studies. The highly sensitive, long-wavelength Atacama Large Millimetre/submillimetre Array (ALMA) can shed light on the problem. This paper presents a survey of M-dwarf stars in the young and nearby Beta Pictoris Moving Group with ALMA at Band 7 (880 μm). From the observational sample, we detect two new sub-mm excesses that likely constitute unresolved debris discs around GJ 2006 A and AT Mic A and model distributions of the disc fractional luminosities and temperatures. From the science sample of 36 M-dwarfs including AU Mic, we find a disc detection rate of 4/36 or 11.1$^{+7.4}_{-3.3}$ per cent that rises to 23.1$^{+8.3}_{-5.5}$ per cent when adjusted for completeness. We conclude that this detection rate is consistent with the detection rate of discs around G- and K-type stars and that the disc properties are also likely consistent with earlier type stars. We additionally conclude that M-dwarf stars are not less likely to host debris discs, but instead their detection requires longer wavelength and higher sensitivity observations than have previously been employed.
more » « less- NSF-PAR ID:
- 10470749
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 526
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 5401-5417
- Size(s):
- ["p. 5401-5417"]
- Sponsoring Org:
- National Science Foundation
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