arXiv subjects

Jonathan Irwin

Publications and source records attributed to Jonathan Irwin.

At least 37 records · Page 2Linked to original sources

arXiv2018

TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844

Data from the newly-commissioned \textit{Transiting Exoplanet Survey Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of $1.32\pm 0.02$ $R_\oplus$ and orbits the star every 11 hours. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough ($I=11.9$, $K=9.1$) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy.

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arXiv2018

New rotation period measurements for M dwarfs in the southern hemisphere: an abundance of slowly rotating, fully convective stars

Stellar rotation periods are valuable both for constraining models of angular momentum loss and for under- standing how magnetic features impact inferences of exoplanet parameters. Building on our previous work in the northern hemisphere, we have used long-term, ground-based photometric monitoring from the MEarth Observatory to measure 234 rotation periods for nearby, southern hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS 1140, and Proxima Centauri. We find excellent agreement between our data and K2 photometry for the overlapping subset. Amongst the sample of stars with the highest quality datasets, we recover periods in 66%; as the length of the dataset increases, our recovery rate approaches 100%. The longest rotation periods we detect are around 140 days, which we suggest represent the periods that are reached when M dwarfs are as old as the local thick disk (about 9 Gyr).

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arXiv2018

The warm Neptunes around HD 106315 have low stellar obliquities

We present the obliquity of the warm Neptune HD 106315c measured via a series of spectroscopic transit observations. HD 106315c is a 4.4 REarth warm Neptune orbiting a moderately rotating late F-star with a period of 21.05 days. HD 106315 also hosts a 2.5 REarth super-Earth on a 9.55 day orbit. Our Doppler tomographic analyses of four transits observed by the Magellan/MIKE, HARPS, and TRES facilities find HD 106315c to be in a low stellar obliquity orbit, consistent with being well aligned with the spin axis of the host star at lambda = -10 +3.6/-3.8 deg. We suggest, via dynamical N-body simulations, that the two planets in the system must be co-planar, and thus are both well aligned with the host star. HD 106315 is only the fourth warm Neptune system with obliquities measured. All warm Neptune systems have been found in well aligned geometries, consistent with the interpretation that these systems are formed in-situ in the inner protoplanetary disk, and also consistent with the majority of Kepler multi-planet systems that are in low obliquity orbits. With a transit depth of 1.02 mmag, HD 106315c is among the smallest planets to have been detected in transit spectroscopy, and we discuss its detection in the context of TESS and the next generations of spectrographs.

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arXiv2018

LHS 1610A: A Nearby Mid-M Dwarf with a Companion That is Likely A Brown Dwarf

We present the spectroscopic orbit of LHS 1610A, a newly discovered single-lined spectroscopic binary with a trigonometric distance placing it at 9.9 pm 0.2 pc. We obtained spectra with the TRES instrument on the 1.5m Tillinghast Reflector at the Fred Lawrence Whipple Observatory located on Mt. Hopkins in AZ. We demonstrate the use of the TiO molecular bands at 7065 -- 7165 Angstroms to measure radial velocities and achieve an average estimated velocity uncertainty of 28 m/s. We measure the orbital period to be 10.6 days and calculate a minimum mass of 44.8 pm 3.2 Jupiter masses for the secondary, indicating that it is likely a brown dwarf. We place an upper limit to 3 sigma of 2500 K on the effective temperature of the companion from infrared spectroscopic observations using IGRINS on the 4.3m Discovery Channel Telescope. In addition, we present a new photometric rotation period of 84.3 days for the primary star using data from the MEarth-South Observatory, with which we show that the system does not eclipse.

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arXiv2017

A Multi-Planet System Transiting the $V$ = 9 Rapidly Rotating F-Star HD 106315

We report the discovery of a multi-planet system orbiting HD 106315, a rapidly rotating mid F-type star, using data from the K2 mission. HD 106315 hosts a $2.51\pm0.12\,R_\oplus$ sub-Neptune in a 9.5 day orbit, and a $4.31_{-0.27}^{+0.24}\,R_\oplus$ super-Neptune in a 21 day orbit. The projected rotational velocity of HD 106315 (12.9 km s$^{-1}$) likely precludes precise measurements of the planets' masses, but could enable a measurement of the sky-projected spin-orbit obliquity for the outer planet via Doppler tomography. The eccentricities of both planets were constrained to be consistent with 0, following a global modeling of the system that includes a Gaia distance and dynamical arguments. The HD 106315 system is one of few multi-planet systems hosting a Neptune-sized planet for which orbital obliquity measurements are possible, making it an excellent test-case for formation mechanisms of warm-Neptunian systems. The brightness of the host star also makes HD 106315 c a candidate for future transmission spectroscopic follow-up studies.

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arXiv2016

The H$α$ emission of nearby M dwarfs and its relation to stellar rotation

The high-energy emission from low-mass stars is mediated by the magnetic dynamo. Although the mechanisms by which fully convective stars generate large-scale magnetic fields are not well understood, it is clear that, as for solar-type stars, stellar rotation plays a pivotal role. We present 270 new optical spectra of low-mass stars in the Solar Neighborhood. Combining our observations with those from the literature, our sample comprises 2202 measurements or non-detections of H$α$ emission in nearby M dwarfs. This includes 466 with photometric rotation periods. Stars with masses between 0.1 and 0.6 solar masses are well-represented in our sample, with fast and slow rotators of all masses. We observe a threshold in the mass-period plane that separates active and inactive M dwarfs. The threshold coincides with the fast-period edge of the slowly rotating population, at approximately the rotation period at which an era of rapid rotational evolution appears to cease. The well- defined active/inactive boundary indicates that H$α$ activity is a useful diagnostic for stellar rotation period, e.g. for target selection for exoplanet surveys, and we present a mass-period relation for inactive M dwarfs. We also find a significant, moderate correlation between $L_{\mathrm{H}α}/L_{\mathrm{bol}}$ and variability amplitude: more active stars display higher levels of photometric variability. Consistent with previous work, our data show that rapid rotators maintain a saturated value of $L_{\mathrm{H}α}/L_{\mathrm{bol}}$. Our data also show a clear power-law decay in $L_{\mathrm{H}α}/L_{\mathrm{bol}}$ with Rossby number for slow rotators, with an index of $-1.7 \pm 0.1$.

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arXiv2016

Results from the 2014 November 15th multi-chord stellar occultation by the TNO (229762) 2007 UK$_{126}$

We present results derived from the first multi-chord stellar occultation by the trans-Neptunian object (229762) 2007 UK$_{126}$, observed on 2014 November 15. The event was observed by the Research and Education Collaborative Occultation Network (RECON) project and International Occultation Timing Association (IOTA) collaborators throughout the United States. Use of two different data analysis methods obtain a satisfactory fit to seven chords, yelding an elliptical fit to the chords with an equatorial radius of $R=338_{-10} ^{+15}$ km and equivalent radius of $R_{eq}=319_{-7} ^{+14}$ km. A circular fit also gives a radius of $R=324_{-23} ^{+30}$ km. Assuming that the object is a Maclaurin spheroid with indeterminate aspect angle, and using two published absolute magnitudes for the body, we derive possible ranges for geometric albedo between $p_{V}=0.159_{-0.013} ^{+0.007}$ and $p_{R}=0.189_{-0.015}^{+0.009}$, and for the body oblateness between $ε=0.105_{-0.040} ^{+0.050}$ and $ε=0.118_{-0.048} ^{+0.055}$. For a nominal rotational period of 11.05 h, an upper limit for density of $ρ=1740$ kg~m$^{-3}$ is estimated for the body.

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arXiv2016

Zodiacal Exoplanets in Time (ZEIT) III: A short-period planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association

We confirm and characterize a close-in ($P_{\rm{orb}}$ = 5.425 days), super-Neptune sized ($5.04^{+0.34}_{-0.37}$ Earth radii) planet transiting K2-33 (2MASS J16101473-1919095), a late-type (M3) pre-main sequence (11 Myr-old) star in the Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth (<20 Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planet's properties and constrain the host star's density. We determine \name's bolometric flux and effective temperature from moderate resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise radius (6-7%) and mass (16%) for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 years, and the independence with wavelength rules out stellar variability, or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude it must instead be planetary in origin. The existence of K2-33b suggests close-in planets can form in situ or migrate within $\sim 10$ Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.

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arXiv2016

The Impact of Stellar Rotation on the Detectability of Habitable Planets Around M Dwarfs

Stellar activity and rotation frustrate the detection of exoplanets through the radial velocity technique. This effect is particularly of concern for M dwarfs, which can remain magnetically active for billions of years. We compile rotation periods for late-type stars and for the M dwarf planet-host sample in order to investigate the rotation periods of older field stars across the main sequence. We show that for stars with masses between 0.25 and 0.5 solar masses (M4V to M1V), the stellar rotation period typical of field stars coincides with the orbital periods of planets in the habitable zone. This will pose a fundamental challenge to the discovery and characterization of potentially habitable planets around early M dwarfs. Due to the longer rotation periods reached by mid M dwarfs and the shorter orbital period at which the planetary habitable zone is found, stars with masses between 0.1 and 0.25 solar masses (M6V to M4V) offer better opportunities for the detection of habitable planets via radial velocities.

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arXiv2016

The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood

Rotation is a directly-observable stellar property, and drives magnetic field generation and activity through a magnetic dynamo. Main sequence stars with masses below approximately 0.35Msun (mid-to-late M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insights into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth transit survey, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no period-amplitude relation for stars below 0.25Msun and an anti-correlation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly-rotating stars without Hα emission that nevertheless have strong photometric variability. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the Solar Neighborhood. We use the W space velocities and established age-velocity relations to estimate that stars with P<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs. The period distribution is mass dependent: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods. [Abridged]

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arXiv2015

A rocky planet transiting a nearby low-mass star

M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun -- are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

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arXiv2015

A disintegrating minor planet transiting a white dwarf

White dwarfs are the end state of most stars, including the Sun, after they exhaust their nuclear fuel. Between 1/4 and 1/2 of white dwarfs have elements heavier than helium in their atmospheres, even though these elements should rapidly settle into the stellar interiors unless they are occasionally replenished. The abundance ratios of heavy elements in white dwarf atmospheres are similar to rocky bodies in the Solar system. This and the existence of warm dusty debris disks around about 4% of white dwarfs suggest that rocky debris from white dwarf progenitors' planetary systems occasionally pollute the stars' atmospheres. The total accreted mass can be comparable to that of large asteroids in the solar system. However, the process of disrupting planetary material has not yet been observed. Here, we report observations of a white dwarf being transited by at least one and likely multiple disintegrating planetesimals with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths up to 40% and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star hosts a dusty debris disk and the star's spectrum shows prominent lines from heavy elements like magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides evidence that heavy element pollution of white dwarfs can originate from disrupted rocky bodies such as asteroids and minor planets.

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arXiv2015

An Activity-Rotation Relationship and Kinematic Analysis of Nearby Mid-to-Late-type M Dwarfs

Using spectroscopic observations and photometric light curves of 238 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by H-alpha emission), rotation period, and stellar age. Previous attempts to investigate the relationship between magnetic activity and rotation in these stars were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that vsini measurements probe only rapid rotation). However, the photometric data from MEarth allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from shorter than than one to longer than 100 days). Over all M spectral types that we probe, we find that the presence of magnetic activity is tied to rotation, including for late-type, fully convective M dwarfs. We also find evidence that the fraction of late-type M dwarfs that are active may be higher at longer rotation periods compared to their early-type counterparts, with several active, late-type, slowly rotating stars present in our sample. Additionally, we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active. This potential mismatch suggests that the physical mechanisms that connect stellar rotation to chromospheric heating may be different in fully convective stars. A kinematic analysis suggests that the magnetically active, rapidly rotating stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.

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arXiv2015

UV variability and accretion dynamics in the young open cluster NGC 2264

We explore UV and optical variability signatures for several hundred members of NGC 2264 (3 Myr). We performed simultaneous u- and r-band monitoring over two full weeks with CFHT/MegaCam. About 750 young stars are probed; 40% of them are accreting. Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, especially in the UV. The amount of u-band variability correlates statistically with UV excess in disk-bearing objects, which suggests that accretion and star-disk interaction are the main sources of variability. Cool magnetic spots, several hundred degrees colder than the photosphere and covering from 5 to 30% of the stellar surface, appear to be the leading factor of variability for the non-accreting stars. In contrast, accretion spots, a few thousand degrees hotter than the photosphere and covering a few percent of the stellar surface, best reproduce the variability of accreting objects. The color behavior is also found to be different between accreting and non-accreting stars. Typical variability amplitudes for accreting members rapidly increase from r to u, which indicates a much stronger contrast at short wavelengths; a lower color dependence in the amplitudes is instead measured for diskless stars. We find that u-band variability on hour timescales is typically about 10% of the peak-to-peak variability on day timescales, while longer term (years) variability is consistent with amplitudes measured over weeks. We conclude that for both accreting and non-accreting stars, the mid-term rotational modulation by spots is the leading timescale for a variability of up to several years. In turn, this suggests that the accretion process is essentially stable over years, although it exhibits low-level shorter term variations in single accretion events.

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arXiv2015

An empirical calibration to estimate cool dwarf fundamental parameters from H-band spectra

Interferometric radius measurements provide a direct probe of the fundamental parameters of M dwarfs, but is within reach for only a limited sample of nearby, bright stars. We use interferometrically-measured radii, bolometric luminosities, and effective temperatures to develop new empirical calibrations based on low-resolution, near-infrared spectra. We use H-band Mg and Al features to derive calibrations for effective temperature, radius and log luminosity; the standard deviations in the residuals of our best fits are, respectively, 73K, 0.027Rsun, and 0.049 dex (11% error on luminosity). These relationships are valid for mid K to mid M dwarf stars, roughly corresponding to temperatures between 3100 and 4800K. We apply our calibrations to M dwarfs targeted by the MEarth transiting planet survey and to the cool Kepler Objects of Interest (KOIs). We independently validate our calibrations by demonstrating a clear relationship between our inferred parameters and the absolute K magnitudes of the MEarth stars, and we identify objects with magnitudes too bright for their estimated luminosities as candidate multiple systems. We also use our inferred luminosities to address the applicability of near-infrared metallicity calibrations to mid and late M dwarfs. The temperatures we infer for the KOIs agree remarkably well with those from the literature; however, our stellar radii are systematically larger than those presented in previous works that derive radii from model isochrones. This results in a mean planet radius that is 15% larger than one would infer using the stellar properties from recent catalogs. Our results confirm those of previous in-depth studies of Kepler-42, Kepler-45, and Kepler-186.

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arXiv2014

Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry

We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in order to directly probe the accretion process from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40% are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha, UV and IR excesses). The remaining non-accreting members define the (photospheric+chromospheric) reference UV emission level over which flux excess is detected and measured. We revise the membership status of cluster members based on UV accretion signatures and report a new population of 50 CTTS candidates. A large range of UV excess is measured for the CTTS population, varying from a few 0.1 to 3 mag. We convert these values to accretion luminosities and obtain mass accretion rates ranging from 1e-10 to 1e-7 Mo/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6sigma correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for upper limits, yields M_acc $\propto$ M^{1.4+/-0.3}. At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, much smaller than the observed spread. We suggest that a non-negligible age spread across the cluster may effectively contribute to the observed spread in accretion rates at a given mass. In addition, different accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-flow accretion) may be associated to different M_acc regimes.

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arXiv2013

Near-infrared Metallicities, Radial Velocities and Spectral Types for 447 Nearby M Dwarfs

We present metallicities, radial velocities and near-infrared spectral types for 447 M dwarfs determined from moderate resolution (R~2000) near-infrared (NIR) spectra obtained with IRTF/SpeX. These M dwarfs are targets of the MEarth Survey, a transiting planet survey searching for super Earths around mid-to-late M dwarfs within 33pc. We present NIR spectral types and new IRTF spectral templates in the Z, J, H and K-bands, created using M dwarfs with near-solar metallicities. We developed two spectroscopic distance calibrations that use NIR spectral type or an index based on the curvature of the K-band continuum. Our distance calibration has a scatter of 14%. We searched 27 NIR spectral lines and 10 spectral indices for metallicity sensitive features, taking into account correlated noise in our estimates of the errors on these parameters. We calibrated our relation using 36 M dwarfs in common proper pairs with an F, G or K-type star of known metallicity. We validated the physical association of these pairs using proper motions, radial velocities and spectroscopic distance estimates. Our resulting metallicity calibration uses the sodium doublet at 2.2um as the sole indicator for metallicity. It has an accuracy of 0.12dex inferred from the scatter between the metallicities of the primaries and the estimated metallicities of the secondaries. Our relation is valid for NIR spectral types from M1V to M5V and for -1.0<[Fe/H]<+0.35dex. We present a new metallicity relation using J-H and J-K colors that directly relates two observables: the distance from the M dwarf main sequence and equivalent width of the sodium line at 2.2um. We measured radial velocities by modeling telluric features to determine the absolute wavelength calibration of our spectra, and used M dwarf binaries, observations at different epochs, and comparison to precisely measured radial velocities to demonstrate 4.4km/s accuracy.

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arXiv2013

Constraints on Planet Occurrence around Nearby Mid-to-Late M Dwarfs from the MEarth Project

The MEarth Project is a ground-based photometric survey to find planets transiting the closest and smallest main-sequence stars. In its first four years, MEarth discovered one transiting exoplanet, the 2.7 Earth radius planet GJ1214b. Here, we answer an outstanding question: in light of the bounty of small planets transiting small stars uncovered by the Kepler mission, should MEarth have found more than just one planet so far? We estimate MEarth's ensemble sensitivity to exoplanets by performing end-to-end simulations of 1.25 million observations of 988 nearby mid-to-late M dwarfs, gathered by MEarth between October 2008 and June 2012. For 2-4 Earth radius planets, we compare this sensitivity to results from Kepler and find that MEarth should have found planets at a rate of 0.05 - 0.36 planets/year in its first four years. As part of this analysis, we provide new analytic fits to the Kepler early M dwarf planet occurrence distribution. When extrapolating between Kepler's early M dwarfs and MEarth's mid-to-late M dwarfs, we find that assuming the planet occurrence distribution stays fixed with respect to planetary equilibrium temperature provides a good match to our detection of a planet with GJ1214b's observed properties. For larger planets, we find that the warm (600-700K), Neptune-sized (4 Earth radius) exoplanets that transit early M dwarfs like Gl436 and GJ3470 occur at a rate of <0.15/star (at 95% confidence) around MEarth's later M dwarf targets. We describe a strategy with which MEarth can increase its expected planet yield by 2.5X without new telescopes, by shifting its sensitivity toward the smaller and cooler exoplanets that Kepler has demonstrated to be abundant.

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