PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs – the most numerous stars of the Galaxy – were left out of these analyses and no calibration of their Ca ii H and K emission to an R'HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M dwarfs, and by evaluating the relationship between R'HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R'HK. We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R'HK index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the activity correlates with its mean level and stars with higher R'HK indexes show larger R'HK variability, as previously observed for earlier spectral types.

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Searching for a dusty cometary belt around TRAPPIST-1 with ALMA

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa266 ◽

2020 ◽

Vol 492 (4) ◽

pp. 6067-6073 ◽

Cited By ~ 2

Author(s):

S Marino ◽

M C Wyatt ◽

G M Kennedy ◽

M Kama ◽

L Matrà ◽

...

Keyword(s):

Liquid Water ◽

Kuiper Belt ◽

Dust Emission ◽

Planetary Systems ◽

Dynamical Instability ◽

Solid Mass ◽

Low Mass Stars ◽

Upper Limit ◽

Low Mass ◽

Collisional Evolution

ABSTRACT Low-mass stars might offer today the best opportunities to detect and characterize planetary systems, especially those harbouring close-in low-mass temperate planets. Among those stars, TRAPPIST-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of TRAPPIST-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23 µJy if the belt is smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ∼10−5 to 10−2 M⊕, which are expected after 8 Gyr of collisional evolution unless the system was born with a >20 M⊕ belt of 100 km-sized planetesimals beyond 40 au or suffered a dynamical instability. This 20 M⊕ mass upper limit is comparable to the combined mass in TRAPPIST-1 planets, thus it is possible that most of the available solid mass in this system was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that a belt born with a mass ≳1 M⊕ in 100 km-sized planetesimals could explain its putative outer belt at 30 au. We recommend that future characterizations of debris discs around low-mass stars should focus on nearby and young systems if possible.

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Rapid Formation of Super-Earths Around Low-Mass Stars

10.5194/epsc2020-155 ◽

2020 ◽

Author(s):

Brianna Zawadzki

Keyword(s):

Density Profile ◽

Surface Density ◽

Planet Formation ◽

Secondary Process ◽

Sufficient Information ◽

Initial Surface ◽

Low Mass Stars ◽

Dwarf Planets ◽

Low Mass ◽

M Dwarf

<p>NASA's TESS mission is expected to discover hundreds of M dwarf planets. However, few studies focus on how planets form around low-mass stars. We aim to better characterize the formation process of M dwarf planets to fill this gap and aid in the interpretation of TESS results. We use six sets of N-body planet formation simulations which vary in whether a gas disc is present, initial range of embryo semi-major axes, and initial solid surface density profile. Each simulation begins with 147 equal-mass embryos around a 0.2 solar mass star and runs for 100 Myr. We find that planets form rapidly, with most collisions occurring within the first 1 Myr. The presence of a gas disc reduces the final number of planets relative to a gas-free environment and causes planets to migrate inward. Because planet formation occurs significantly faster than the disc lifetime, super-Earths have plenty of time to accrete extended gaseous envelopes, though these may later be removed by collisions or a secondary process like photo-evaporation. In addition, we find that the final distribution of planets does not retain a memory of the slope of the initial surface density profile, regardless of whether or not a gas disc is present. Thus, our results suggest that present-day observations are unlikely to provide sufficient information to accurately reverse-engineer the initial distribution of solids.</p>

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An eclipsing M-dwarf close to the hydrogen burning limit from NGTS

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2513 ◽

2020 ◽

Vol 498 (3) ◽

pp. 3115-3124

Author(s):

Jack S Acton ◽

Michael R Goad ◽

Sarah L Casewell ◽

Jose I Vines ◽

Matthew R Burleigh ◽

...

Keyword(s):

Binary System ◽

Binary Systems ◽

Binary Star ◽

Mass Ratio ◽

Low Mass Stars ◽

Hydrogen Burning ◽

Low Mass ◽

Global Modelling ◽

Relationship Of ◽

M Dwarf

ABSTRACT We present the discovery of NGTS J0930−18, an extreme mass ratio eclipsing M-dwarf binary system with an early M-dwarf primary and a late M-dwarf secondary close to the hydrogen burning limit. Global modelling of photometry and radial velocities reveals that the secondary component (NGTS J0930−18 B) has a mass of M* = $0.0818 ^{+0.0040}_{-0.0015}$ M⊙ and radius of R* = $0.1059 ^{+0.0023}_{-0.0021}$ R⊙, making it one of the lowest mass stars with direct mass and radius measurements. With a mass ratio of q = $0.1407 ^{+0.0065}_{-0.017}$, NGTS J0930−18 has the lowest mass ratio of any known eclipsing M-dwarf binary system, posing interesting questions for binary star formation and evolution models. The mass and radius of NGTS J0930−18 B is broadly consistent with stellar evolutionary models. NGTS J0930−18 B lies in the sparsely populated mass radius parameter space close to the substellar boundary. Precise measurements of masses and radii from single lined eclipsing binary systems of this type are vital for constraining the uncertainty in the mass–radius relationship – of importance due to the growing number of terrestrial planets being discovered around low-mass stars.

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Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. IV. Updated Properties for 86 Cool Dwarfs Observed during Campaigns 1–17

The Astronomical Journal ◽

10.3847/1538-3881/ab2895 ◽

2019 ◽

Vol 158 (2) ◽

pp. 87 ◽

Cited By ~ 6

Author(s):

Courtney D. Dressing ◽

Kevin Hardegree-Ullman ◽

Joshua E. Schlieder ◽

Elisabeth R. Newton ◽

Andrew Vanderburg ◽

...

Keyword(s):

Planetary Systems ◽

Low Mass Stars ◽

Low Mass ◽

Cool Dwarfs

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2MASS J15460752−6258042: a mid-M dwarf hosting a prolonged accretion disc

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa689 ◽

2020 ◽

Vol 494 (1) ◽

pp. 62-68 ◽

Cited By ~ 5

Author(s):

Jinhee Lee ◽

Inseok Song ◽

Simon Murphy

Keyword(s):

Accretion Rate ◽

Spectral Energy Distribution ◽

Spectral Energy ◽

Velocity Measurements ◽

M Dwarfs ◽

Low Mass Stars ◽

Infrared Excess ◽

Low Mass ◽

M Dwarf ◽

Gaia Dr2

ABSTRACT We report the discovery of the oldest (∼55 Myr) mid-M type star known to host ongoing accretion. 2MASS J15460752–6258042 (2M1546, spectral type M5, 59.2 pc) shows spectroscopic signs of accretion such as strong H α, He i, and [O i] emission lines, from which we estimate an accretion rate of ∼10−10 M⊙ yr−1. Considering the clearly detected infrared excess in all WISE bands, the shape of its spectral energy distribution (SED) and its age, we believe that the star is surrounded by a transitional disc, clearly with some gas still present at inner radii. The position and kinematics of the star from Gaia DR2 and our own radial-velocity measurements suggest membership in the nearby ∼55 Myr-old Argus moving group. At only 59 pc from Earth, 2M1546 is one of the nearest accreting mid-M dwarfs, making it an ideal target for studying the upper limit on the lifetimes of gas-rich discs around low-mass stars.

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Diverse outcomes of planet formation and composition around low-mass stars and brown dwarfs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3007 ◽

2019 ◽

Cited By ~ 5

Author(s):

Y Miguel ◽

A Cridland ◽

C W Ormel ◽

J J Fortney ◽

S Ida

Keyword(s):

Planet Formation ◽

Planetary Systems ◽

Brown Dwarfs ◽

Population Synthesis ◽

Optimal Conditions ◽

Low Mass Stars ◽

Water Fraction ◽

Low Mass ◽

M Stars ◽

Mass Form

Abstract The detection of Earth-size exoplanets around low-mass stars –in stars such as Proxima Centauri and TRAPPIST-1– provide an exceptional chance to improve our understanding of the formation of planets around M stars and brown dwarfs. We explore the formation of such planets with a population synthesis code based on a planetesimal-driven model previously used to study the formation of the Jovian satellites. Because the discs have low mass and the stars are cool, the formation is an inefficient process that happens at short periods, generating compact planetary systems. Planets can be trapped in resonances and we follow the evolution of the planets after the gas has dissipated and they undergo orbit crossings and possible mergers. We find that formation of planets above Mars mass and in the planetesimal accretion scenario, is only possible around stars with masses M⋆ ≥ 0.07Msun and discs of Mdisc ≥ 10−2 Msun. We find that planets above Earth-mass form around stars with masses larger than 0.15 Msun, while planets larger than 5 M⊕ do not form in our model, even not under the most optimal conditions (massive disc), showing that planets such as GJ 3512b form with another, more efficient mechanism. Our results show that the majority of planets form with a significant water fraction; that most of our synthetic planetary systems have 1, 2 or 3 planets, but planets with 4,5,6 and 7 planets are also common, confirming that compact planetary systems with many planets should be a relatively common outcome of planet formation around small stars.

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Fast photometric variability of very low mass stars in IC 348: detection of superflare in an M dwarf

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3574 ◽

2020 ◽

Vol 500 (4) ◽

pp. 5106-5116

Author(s):

Samrat Ghosh ◽

Soumen Mondal ◽

Somnath Dutta ◽

Ramkrishna Das ◽

Santosh Joshi ◽

...

Keyword(s):

Light Curves ◽

M Dwarfs ◽

Low Mass Stars ◽

Period Range ◽

Photometric Variability ◽

Star Forming ◽

Low Mass ◽

M Dwarf ◽

Variability Study ◽

Optical Flare

ABSTRACT We present here optical I-band photometric variability study down to ≃19 mag of a young (∼2–3 Myr) star-forming region IC 348 in the Perseus molecular cloud. We aim to explore the fast rotation (in the time-scales of hours) in very low-mass stars including brown dwarfs (BDs). From a sample of 177 light curves using our new I-band observations, we detect new photometric variability in 22 young M dwarfs including 6 BDs, which are bonafide members in IC 348 and well characterized in the spectral type of M dwarfs. Out of 22 variables, 11 M dwarfs including one BD show hour-scale periodic variability in the period range 3.5–11 h and rest are aperiodic in nature. Interestingly, an optical flare is detected in a young M2.75 dwarf in one night data on 2016 December 20. From the flare light curve, we estimate the emitted flared energy of 1.48 × 1035 erg. The observed flared energy with an uncertainty of tens of per cent is close to the superflare range (∼1034 erg), which is rarely observed in active M dwarfs.

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A closer look at the transition between fully convective and partly radiative low-mass stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834062 ◽

2018 ◽

Vol 619 ◽

pp. A177 ◽

Cited By ~ 2

Author(s):

Isabelle Baraffe ◽

Gilles Chabrier

Keyword(s):

Stellar Evolution ◽

Structural Changes ◽

Main Sequence ◽

Energy Transport ◽

M Dwarfs ◽

Low Mass Stars ◽

Mixing Processes ◽

Nuclear Burning ◽

Low Mass ◽

M Dwarf

Recently, an analysis of Gaia Data Release 2 revealed a gap in the mid-M dwarf main sequence. The authors suggested the feature is linked to the onset of full convection in M dwarfs. Following the announcement of this discovery, an explanation has been proposed based on standard stellar evolution models. In this paper we re-examine this explanation. We confirm that nuclear burning and mixing processes of 3He provide the best explanation for the observed feature. We also find that a change in the energy transport from convection to radiation does not induce structural changes that could be visible. Regarding the very details of the process, however, we disagree with the details of the published explanation and propose an alternative.

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