arXiv subjects

Simon Hodgkin

Publications and source records attributed to Simon Hodgkin.

At least 37 records · Page 2Linked to original sources

arXiv2018

Low-mass eclipsing binaries in the WFCAM Transit Survey: the persistence of the M-dwarf radius inflation problem

We present the characterization of 5 new short-period low-mass eclipsing binaries from the WFCAM Transit Survey. The analysis was performed by using the photometric WFCAM J-mag data and additional low- and intermediate-resolution spectroscopic data to obtain both orbital and physical properties of the studied sample. The light curves and the measured radial velocity curves were modeled simultaneously with the JKTEBOP code, with Markov chain Monte Carlo simulations for the error estimates. The best-model fit have revealed that the investigated detached binaries are in very close orbits, with orbital separations of $2.9 \leq a \leq 6.7$ $R_{\odot}$ and short periods of $0.59 \leq P_{\rm orb} \leq 1.72$ d, approximately. We have derived stellar masses between $0.24$ and $0.72$ $M_{\odot}$ and radii ranging from $0.42$ to $0.67$ $R_{\odot}$. The great majority of the LMEBs in our sample has an estimated radius far from the predicted values according to evolutionary models. The components with derived masses of $M < 0.6$ $M_{\odot}$ present a radius inflation of $\sim$$9\%$ or more. This general behavior follows the trend of inflation for partially-radiative stars proposed previously. These systems add to the increasing sample of low-mass stellar radii that are not well-reproduced by stellar models. They further highlight the need to understand the magnetic activity and physical state of small stars. Missions like TESS will provide many such systems to perform high-precision radius measurements to tightly constrain low-mass stellar evolution models.

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arXiv2018

Discovery of New Dipper Stars with K2: A Window into the Inner Disk Region of T Tauri Stars

In recent years a new class of Young Stellar Object has been defined, referred to as dippers, where large transient drops in flux are observed. These dips are too large to be attributed to stellar variability, last from hours to days and can reduce the flux of a star by 10-50\%. This variability has been attributed to occultations by warps or accretion columns near the inner edge of circumstellar disks. Here we present 95 dippers in the Upper Scorpius association and $ρ$ Ophiuchus cloud complex found in K2 Campaign 2 data using supervised machine learning with a Random Forest classifier. We also present 30 YSOs that exhibit brightening events on the order of days, known as bursters. Not all dippers and bursters are known members, but all exhibit infrared excesses and are consistent with belonging to either of the two young star forming regions. We find 21.0 $\pm$ 5.5\% of stars with disks are dippers for both regions combined. Our entire dipper sample consists only of late-type (KM) stars, but we show that biases limit dipper discovery for earlier spectral types. Using the dipper properties as a proxy, we find that the temperature at the inner disk edge is consistent with interferometric results for similar and earlier type stars.

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arXiv2014

Detection of the secondary eclipse of WASP-10b in the Ks-band

WASP-10b, a non-inflated hot Jupiter, was discovered around a K-dwarf in a near circular orbit ($\sim $$0.06$). Since its discovery in 2009, different published parameters for this system have led to a discussion about the size, density, and eccentricity of this exoplanet. In order to test the hypothesis of a circular orbit for WASP-10b, we have observed its secondary eclipse in the Ks-band, where the contribution of planetary light is high enough to be detected from the ground. Observations were performed with the OMEGA2000 instrument at the 3.5-meter telescope at Calar Alto (Almería, Spain), in staring mode during 5.4 continuous hours, with the telescope defocused, monitoring the target during the expected secondary eclipse. A relative light curve was generated and corrected from systematic effects, using the Principal Component Analysis (PCA) technique. The final light curve was fitted using a transit model to find the eclipse depth and a possible phase shift. The best model obtained from the Markov Chain Monte Carlo analysis resulted in an eclipse depth of $ΔF$ of $0.137\%^{+0.013\%}_{-0.019\%}$ and a phase offset of $Δϕ$ of $-0.0028^{+0.0005}_{-0.0004}$. The eclipse phase offset derived from our modeling has systematic errors that were not taken into account and should not be considered as evidence of an eccentric orbit. The offset in phase obtained leads to a value for $|e\cosω|$ of $0.0044$. The derived eccentricity is too small to be of any significance.

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arXiv2013

CoRoT 223992193: A new, low-mass, pre-main sequence eclipsing binary with evidence of a circumbinary disk

We present the discovery of CoRoT 223992193, a double-lined, detached eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the CoRoT space mission during a 23-day observation of the 3 Myr old NGC 2264 star-forming region. Using multi-epoch optical and near-IR follow-up spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William Herschel Telescope we obtain a full orbital solution and derive the fundamental parameters of both stars by modelling the light curve and radial velocity data. The orbit is circular and has a period of $3.8745745 \pm 0.0000014$ days. The masses and radii of the two stars are $0.67 \pm 0.01$ and $0.495 \pm 0.007$ $M_{\odot}$ and $1.30 \pm 0.04$ and $1.11 ~^{+0.04}_{-0.05}$ $R_{\odot}$, respectively. This system is a useful test of evolutionary models of young low-mass stars, as it lies in a region of parameter space where observational constraints are scarce; comparison with these models indicates an apparent age of $\sim$3.5-6 Myr. The systemic velocity is within $1σ$ of the cluster value which, along with the presence of lithium absorption, strongly indicates cluster membership. The CoRoT light curve also contains large-amplitude, rapidly evolving out-of-eclipse variations, which are difficult to explain using starspots alone. The system's spectral energy distribution reveals a mid-infrared excess, which we model as thermal emission from a small amount of dust located in the inner cavity of a circumbinary disk. In turn, this opens up the possibility that some of the out-of-eclipse variability could be due to occultations of the central stars by material located at the inner edge or in the central cavity of the circumbinary disk.

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arXiv2012

Photometric Science Alerts from Gaia

Gaia is the cornerstone mission of the European Space Agency. From late 2013 it will start collecting superb astrometric, photometric and spectroscopic data for around a billion of stars of our Galaxy. While surveying the whole sky down to V=20mag Gaia will be detecting transients and anomalous behaviour of objects, providing near-real-time alerts to the entire astronomical community. Gaia should detected about 6000 supernovae, 1000 microlensing events and many other interesting types of transients. Thanks to its on-board low-dispersion spectrograph the classification of transients will be robust, assuring low false-alert rate. We describe the operation of the Photometric Science Alerts system, outline the scientific possibilities and conclude with an invitation to collaborate in the ground-based follow-up Gaia alerts during the early months of the mission when the outcome of the alerting pipeline needs to be verified.

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arXiv2011

Around Gaia Alerts in 20 questions

Gaia is a European Space Agency (ESA) astrometry space mission, and a successor to the ESA Hipparcos mission. Gaia's main goal is to collect high-precision astrometric data (i.e. positions, parallaxes, and proper motions) for the brightest 1 billion objects in the sky. These data, complemented with multi-band, multi-epoch photometric and spectroscopic data collected from the same observing platform, will allow astronomers to reconstruct the formation history, structure, and evolution of the Galaxy. Gaia will observe the whole sky for 5 years, providing a unique opportunity for the discovery of large numbers of transient and anomalous events, e.g. supernovae, novae and microlensing events, GRB afterglows, fallback supernovae, and other theoretical or unexpected phenomena. The Photometric Science Alerts team has been tasked with the early detection, classification and prompt release of anomalous sources in the Gaia data stream. In this paper, we discuss the challenges we face in preparing to use Gaia to search for transient phenomena at optical wavelengths.

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arXiv2011

Deep wide-field near-infrared survey of the Carina Nebula

(abbreviated) We used HAWK-I at the ESO VLT to produce a near-infrared survey of the Carina Nebula that is deep enough to detect the full low-mass stellar population. The results of a recent deep X-ray survey are used to distinguish between young stars in Carina and background contaminants. We find that the ages of the low-mass stars (derived from color-magnitude diagrams of the invidual cluster in the Carina Nebula) agree with previous age estimates for the massive stars. About 3200 of the X-ray selected stars have masses >= 1 Msun; this number is in good agreement with extrapolations of the field IMF based on the number of high-mass stars and shows that there is no deficit of low-mass stars. The near-infrared excess fractions for the stellar populations in Carina are lower than typical for other, less massive clusters of similar age, suggesting a faster timescale of circumstellar disk dispersal than in the more quiescent regions, most likely due to the very high level of massive star feedback. Narrow-band images reveal six molecular hydrogen jets. However, none of the optical HH objects shows molecular hydrogen emission, suggesting that the jet-driving protostars are located very close to the edges of the globules in which they are embedded. This adds strong support to the scenario that their formation was triggered by the advancing ionization fronts.

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arXiv2011

Near-Infrared properties of the X-ray emitting young stellar objects in the Carina Nebula

Abbreviated Abstract: The near-infrared study of the Carina Nebula in this paper builds on the results of the Chandra Carina Complex Project (CCCP), that detected 14368 X-ray sources in the 1.4 square-degree survey region, an automatic source classification study that classified 10714 of these as very likely young stars in Carina, and an analysis of their clustering properties. We used HAWK-I at the ESO VLT to conduct a very deep near-IR survey with sub-arcsecond angular resolution, covering about 1280 square-arcminutes. The HAWK-I images reveal more than 600000 individual infrared sources, whereby objects as faint as J ~ 23, H ~ 22, and Ks ~ 21 are detected at S/N >= 3. While less than half of the Chandra X-ray sources have counterparts in the 2MASS catalog, the ~5 mag deeper HAWK-I data reveal infrared counterparts to 6636 (= 88.8%) of the 7472 Chandra X-ray sources in the HAWK-I field. We analyze near-infrared color-color and color-magnitude diagrams to derive information about the extinctions, infrared excesses, ages, and masses of the X-ray selected objects. The near-infrared properties agree well with the results of the automatic X-ray source classification. The shape of the K-band luminosity function of the X-ray selected Carina members agrees well with that derived for the Orion Nebula Cluster, suggesting that, down to the X-ray detection limit around 0.5-1 Msun, the shape of the IMF in Carina is consistent with that in Orion (and thus the field IMF). The fraction of stars with near-infrared excesses is rather small, <=10%, but shows considerable variations between individual parts of the complex. The distribution of extinctions for the diskless stars ranges from ~1.6 mag to ~6.2 mag (central 80th percentile), clearly showing a considerable range of differential extinction between individual stars in the complex.

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arXiv2011

A Catalog of Chandra X-ray Sources in the Carina Nebula

We present a catalog of ~14,000 X-ray sources observed by the ACIS instrument on the Chandra X-ray Observatory within a 1.42 square degree survey of the Great Nebula in Carina, known as the Chandra Carina Complex Project (CCCP). This study appears in a Special Issue of the ApJS devoted to the CCCP. Here, we describe the data reduction and analysis procedures performed on the X-ray observations, including calibration and cleaning of the X-ray event data, point source detection, and source extraction. The catalog appears to be complete across most of the field to an absorption-corrected total-band luminosity of ~10^{30.7} erg/s for a typical low-mass pre-main sequence star. Counterparts to the X-ray sources are identified in a variety of visual, near-infrared, and mid-infrared surveys. The X-ray and infrared source properties presented here form the basis of many CCCP studies of the young stellar populations in Carina.

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arXiv2011

Rotation periods for very low mass stars in Praesepe

We investigate the rotation periods of fully convective very low mass stars (VLM, M<0.3 Msol), with the aim to derive empirical constraints for the spindown due to magnetically driven stellar winds. Our analysis is based on a new sample of rotation periods in the main-sequence cluster Praesepe (age 600 Myr). From photometric lightcurves obtained with the Isaac Newton Telescope, we measure rotation periods for 49 objects, among them 26 in the VLM domain. This enlarges the period sample in this mass and age regime by a factor of 6. Almost all VLM objects in our sample are fast rotators with periods <2.5 d, in contrast to the stars with M>0.6 Msol in this cluster which have periods of 7-14 d. Thus, we confirm that the period-mass distribution in Praesepe exhibits a radical break at M~0.3-0.6 Msol. Our data indicate a positive period-mass trend in the VLM regime, similar to younger clusters. In addition, the scatter of the periods increases with mass. For the M>0.3 Msol objects in our sample the period distribution is probably affected by binarity. By comparing the Praesepe periods with literature samples in the cluster NGC2516 (age ~150 Myr) we constrain the spindown in the VLM regime. An exponential rotational braking law P ~ exp(t/tau) with a mass-dependent tau is required to reproduce the data. The spindown timescale tau increases steeply towards lower masses; we derive tau~0.5 Gyr for 0.3 Msol and >1 Gyr for 0.1 Msol. These constraints are consistent with the current paradigm of the spindown due to wind braking. We discuss possible physical origins of this behaviour and prospects for future work.

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arXiv2008

The Monitor project: Rotation periods of low-mass stars in M50

We report on the results of a time-series photometric survey of M50 (NGC 2323), a ~130 Myr open cluster, carried out using the CTIO 4m Blanco telescope and Mosaic-II detector as part of the Monitor project. Rotation periods were derived for 812 candidate cluster members over the mass range 0.2 <~ M/Msol <~ 1.1. The rotation period distributions show a clear mass-dependent morphology, statistically indistinguishable from those in NGC 2516 and M35 taken from the literature. Due to the availability of data from three observing runs separated by ~10 and 1 month timescales, we are able to demonstrate clear evidence for evolution of the photometric amplitudes, and hence spot patterns, over the 10 month gap, although we are not able to constrain the timescales for these effects in detail due to limitations imposed by the large gaps in our sampling, preventing use of the phase information.

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arXiv2008

The Monitor project: the search for transits in the open cluster NGC 2362

We present the results of a systematic search for transiting planets in a ~5 Myr open cluster, NGC 2362. We observed ~1200 candidate cluster members, of which ~475 are believed to be genuine cluster members, for a total of ~100 hours. We identify 15 light curves with reductions in flux that pass all our detection criteria, and 6 of the candidates have occultation depths compatible with a planetary companion. The variability in these six light curves would require very large planets to reproduce the observed transit depth. If we assume that none of our candidates are in fact planets then we can place upper limits on the fraction of stars with hot Jupiters (HJs) in NGC 2362. We obtain 99% confidence upper limits of 0.22 and 0.70 on the fraction of stars with HJs (f_p) for 1-3 and 3-10 day orbits, respectively, assuming all HJs have a planetary radius of 1.5R_Jup. These upper limits represent observational constraints on the number of stars with HJs at an age <~10 Myr, when the vast majority of stars are thought to have lost their protoplanetary discs. Finally, we extend our results to the entire Monitor Project, a survey searching young, open clusters for planetary transits, and find that the survey as currently designed should be capable of placing upper limits on f_p near the observed values of f_p in the solar neighbourhood.

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arXiv2007

The Monitor project: Rotation of low-mass stars in NGC 2362 -- testing the disc regulation paradigm at 5 Myr

We report on the results of a time-series photometric survey of NGC 2362, carried out using the CTIO 4m Blanco telescope and Mosaic-II detector as part of the Monitor project. Rotation periods were derived for 271 candidate cluster members over the mass range 0.1 <~ M/Msol <~ 1.2. The rotation period distributions show a clear mass-dependent morphology, qualitatively similar to that in NGC 2264, as would be expected from the age of this cluster. Using models of angular momentum evolution, we show that angular momentum losses over the ~1-5 Myr age range appear to be needed in order to reproduce the evolution of the slowest rotators in the sample from the ONC to NGC 2362, as found by many previous studies. By incorporating Spitzer IRAC mid-IR measurements, we found that 3-4 objects showing mid-IR excesses indicative of the presence of circumstellar discs were all slow rotators, as would be expected in the disc regulation paradigm for early pre-main sequence angular momentum evolution, but this result is not statistically significant at present, given the extremely limited sample size.

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arXiv2007

The Monitor project: tracking the evolution of low mass and pre-main sequence stars

The Monitor project is a large-scale program of photometric and spectroscopic monitoring of young open clusters using telescopes at ESO and other observatories. Its primary goal is to detect and characterise new low-mass eclipsing binaries, and the first three detected systems are discussed here. We derive the masses and radii of the components of each system directly from the light and radial velocity curves, and compare them to the predictions of commonly used theoretical evolutionary models of low-mass stars.

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arXiv2007

The Monitor project: Rotation of low-mass stars in the open cluster NGC 2547

We report on the results of an I-band time-series photometric survey of NGC 2547 using the MPG/ESO 2.2m telescope with WFI, achieving better than 1% photometric precision per data point over 14 <~ I <~ 18. Candidate cluster members were selected from a V vs V-I colour magnitude diagram over 12.5 < V < 24 (covering masses from 0.9 Msol down to below the brown dwarf limit), finding 800 candidates, of which we expect ~330 to be real cluster members, taking into account contamination from the field (which is most severe at the extremes of our mass range). Searching for periodic variations in these gave 176 detections over the mass range 0.1 <~ M/Msol <~ 0.9. The rotation period distributions were found to show a clear mass-dependent morphology, qualitatively intermediate between the distributions obtained from similar surveys in NGC 2362 and NGC 2516, as would be expected from the age of this cluster. Models of the rotational evolution were investigated, finding that the evolution from NGC 2362 to NGC 2547 was qualitatively reproduced (given the uncertainty in the age of NGC 2547) by solid body and core-envelope decoupled models from our earlier NGC 2516 study without need for significant modification.

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arXiv2007

The Monitor project: JW 380 -- a 0.26, 0.15 Msol pre main sequence eclipsing binary in the Orion Nebula Cluster

We report the discovery of a low-mass (0.26 +/- 0.02, 0.15 +/- 0.01 Msol) pre-main-sequence eclipsing binary with a 5.3 day orbital period. JW 380 was detected as part of a high-cadence time-resolved photometric survey (the Monitor project) using the 2.5m Isaac Newton Telescope and Wide Field Camera for a survey of a single field in the Orion Nebula Cluster (ONC) region in V and i bands. The star is assigned a 99 per cent membership probability from proper motion measurements, and radial velocity observations indicate a systemic velocity within 1 sigma of that of the ONC. Modelling of the combined light and radial velocity curves of the system gave stellar radii of 1.19 +0.04 -0.18 Rsol and 0.90 +0.17 -0.03 Rsol for the primary and secondary, with a significant third light contribution which is also visible as a third peak in the cross-correlation functions used to derive radial velocities. The masses and radii appear to be consistent with stellar models for 2-3 Myr age from several authors, within the present observational errors. These observations probe an important region of mass-radius parameter space, where there are currently only a handful of known pre-main-sequence eclipsing binary systems with precise measurements available in the literature.

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arXiv2007

The Monitor project: Rotation of low-mass stars in the open cluster NGC 2516

We report on the results of an i-band time-series photometric survey of NGC 2516 using the CTIO 4m Blanco telescope and 8k Mosaic-II detector, achieving better than 1% photometric precision per data point over 15<~i<~19. Candidate cluster members were selected from a V vs V-I colour magnitude diagram over 16 0.25 days, and the slowest rotators for M<~0.5 M_solar bounded by a line of P \propto M^3, with those for M>~0.5 M_solar following a flatter relation closer to P ~ constant. Models of the rotational evolution were investigated, finding that the evolution of the fastest rotators was well-reproduced by a conventional solid body model with a mass-dependent saturation velocity, whereas core-envelope decoupling was needed to reproduce the evolution of the slowest rotators. None of our models were able to simultaneously reproduce the behaviour of both populations.

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arXiv2006

The Monitor project: Data processing and lightcurve production

We have begun a large-scale photometric survey of nearby open clusters and star-forming regions, the Monitor project, aiming to measure time-series photometry for >10,000 cluster members over >10 sq.deg of sky, to find low-mass eclipsing binary and planet systems. We describe the software pipeline we have developed for this project, showing that we can achieve peak RMS accuracy over the entire data-set of better than 2 mmag using aperture photometry, with RMS < 1% over 4 mag, in data from 2 and 4m class telescopes with wide-field mosaic cameras. We investigate the noise properties of our data, finding correlated `red' noise at the 1 - 1.5 mmag level in bright stars, over transit-like timescales of 2.5 hours. An important source of correlated noise in aperture photometry is image blending, which produces variations correlated with the seeing. We present a simple blend index based on fitting polynomials to these variations, and find that subtracting the fit from the data provides a method to reduce their amplitude, in lieu of using techniques such as point spread function fitting photometry which tackle their cause. Finally, we use the Sysrem algorithm to search for any further systematic effects.

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