arXiv subjects

Markus Rabus

Publications and source records attributed to Markus Rabus.

At least 55 records · Page 3Linked to original sources

arXiv2019

Spitzer Microlensing parallax reveals two isolated stars in the Galactic bulge

We report the mass and distance measurements of two single-lens events from the 2017 Spitzer microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and Spitzer observations. We find that the lens of OGLE-2017-BLG-1254 is a $0.60 \pm 0.03 M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.11~\text{kpc}$, where $D_{\rm LS}$ is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a $0.51^{+0.12}_{-0.10} M_{\odot}$ star with $D_{\rm LS} = 0.40 \pm 0.12~\text{kpc}$ or a $0.38^{+0.13}_{-0.12} M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.19~\text{kpc}$. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published Spitzer finite-source events with the expectations from a Galactic model, we find that the Spitzer sample is in agreement with the probability of finite-source effects occurrence in single lens events.

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arXiv2019

A discontinuity in the $T_{\rm eff}$-radius relation of M-dwarfs

We report on 13 new high-precision measurements of stellar diameters for low-mass dwarfs obtained by means of near-infrared long-baseline interferometry with PIONIER at the Very Large Telescope Interferometer. Together with accurate parallaxes from Gaia DR2, these measurements provide precise estimates for their linear radii, effective temperatures, masses, and luminosities. This allows us to refine the effective temperature scale, in particular towards the coolest M-dwarfs. We measure for late-type stars with enhanced metallicity slightly inflated radii, whereas for stars with decreased metallicity we measure smaller radii. We further show that Gaia DR2 effective temperatures for M-dwarfs are underestimated by $\sim$ 8.2 % and give an empirical $M_{G}$-$T_{\rm eff}$ relation which is better suited for M-dwarfs with $T_{\rm eff}$ between 2600 and 4000 K. Most importantly, we are able to observationally identify a discontinuity in the $T_{\rm eff}$-radius plane, which is likely due to the transition from partially convective M-dwarfs to the fully convective regime. We found this transition to happen between 3200 K and 3340 K, or equivalently for stars with masses $\approx 0.23 M_{\odot}$. We find that in this transition region the stellar radii are in the range from 0.18 to 0.42$R_{\odot}$ for similar stellar effective temperatures.

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arXiv2019

$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf

The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object mass measurement to date, only $\sim$45\% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpc or $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.

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arXiv2019

Towards reliable uncertainties in IR interferometry: The bootstrap for correlated statistical & systematic errors

We propose a method to overcome the usual limitation of current data processing techniques in optical and infrared long-baseline interferometry: most reduction pipelines assume uncorrelated statistical errors and ignore systematics. We use the bootstrap method to sample the multivariate probability density function of the interferometric observables. It allows us to determine the correlations between statistical error terms and their deviation from a Gaussian distribution. In addition, we introduce systematics as an additional, highly correlated error term whose magnitude is chosen to fit the data dispersion. We have applied the method to obtain accurate measurements of stellar diameters for under-resolved stars, i.e. smaller than the angular resolution of the interferometer. We show that taking correlations and systematics has a significant impact on both the diameter estimate and its uncertainty. The robustness of our diameter determination comes at a price: we obtain 4 times larger uncertainties, of a few percent for most stars in our sample.

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arXiv2018

K2-287b: an Eccentric Warm Saturn transiting a G-dwarf

We report the discovery of K2-287b, a Saturn mass planet orbiting a G-dwarf with a period of $P \approx 15$ days. First uncovered as a candidate using K2 campaign 15 data, follow-up photometry and spectroscopy were used to determine a mass of $M_P = 0.317 \pm 0.026$ $M_J$, radius $R_P = 0.833 \pm 0.013$ $R_J$, period $P = 14.893291 \pm 0.000025$ days and eccentricity $e = 0.476 \pm 0.026$. The host star is a metal-rich $V=11.410 \pm 0.129$ mag G dwarf for which we estimate a mass $M_* = 1.056$ $M_\odot$, radius $R_* = 1.07 \pm 0.01$ $R_\odot$, metallicity [Fe/H] = $0.20 \pm 0.05$ and $T_{eff} = 5673 \pm 75$ K. This warm eccentric planet with a time-averaged equilibrium temperature of $T_{eq} \approx 800$ K adds to the small sample of giant planets orbiting nearby stars whose structure is not expected to be affected by stellar irradiation. Follow-up studies on the K2-287 system could help in constraining theories of migration of planets in close-in orbits.

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arXiv2018

HATS-54b-HATS-58Ab: five new transiting hot Jupiters including one with a possible temperate companion

We report the discovery by the HATSouth project of 5 new transiting hot Jupiters (HATS-54b through HATS-58Ab). HATS-54b, HATS-55b and HATS-58Ab are prototypical short period ($P = 2.5-4.2$ days, $R_p\sim1.1-1.2$ $R_J$) hot-Jupiters that span effective temperatures from 1350 K to 1750 K, putting them in the proposed region of maximum radius inflation efficiency. The HATS-58 system is composed of two stars, HATS-58A and HATS-58B, which are detected thanks to Gaia DR2 data and which we account for in the joint modelling of the available data --- with this, we are led to conclude that the hot jupiter orbits the brighter HATS-58A star. HATS-57b is a short-period (2.35-day) massive (3.15 $M_J$) 1.14 $R_J$, dense ($2.65\pm0.21$ g cm$^{-3}$) hot-Jupiter, orbiting a very active star ($2\%$ peak-to-peak flux variability). Finally, HATS-56b is a short period (4.32-day) highly inflated hot-Jupiter (1.7 $R_J$, 0.6 $M_J$), which is an excellent target for future atmospheric follow-up, especially considering the relatively bright nature ($V=11.6$) of its F dwarf host star. This latter exoplanet has another very interesting feature: the radial velocities show a significant quadratic trend. If we interpret this quadratic trend as arising from the pull of an additional planet in the system, we obtain a period of $P_c = 815^{+253}_{-143}$ days for the possible planet HATS-56c, and a minimum mass of $M_c\sin i_c = 5.11 \pm 0.94$ $M_J$. The candidate planet HATS-56c would have a zero-albedo equilibrium temperature of $T_\textrm{eq}=332\pm 50$ K, and thus would be orbiting close to the habitable zone of HATS-56. Further radial-velocity follow-up, especially over the next two years, is needed to confirm the nature of HATS-56c.

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arXiv2018

A Higher Cadence Subsurvey Located in the Galactic Plane

Presently, the Galactic plane receives relatively few observations compared to most of the LSST footprint. While this may address static science, the plane will also represent the highest density of variable Galactic sources. The proper characterization of variability of these sources will benefit greatly from observations at a higher cadence.

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arXiv2018

HD 1397b: a transiting warm giant planet orbiting a V = 7.8 mag sub-giant star discovered by TESS

We report the discovery of a transiting planet first identified as a candidate in Sector 1 of the Transiting Exoplanet Survey Satellite (TESS), and then confirmed with precision radial velocities. HD1397b has a mass of ${\rm M_P}$ = $0.335_{-0.018}^{+0.018}$ ${\rm M_J}$, a radius of ${\rm R_P}$ = $1.021_{-0.014}^{+0.015}$ ${\rm M_J}$, and orbits its bright host star ($V = 7.8$ mag) with an orbital period of $11.53508 \pm 0.00057 $ d, on a moderately eccentric orbit ($e$ = 0.210 $\pm$ 0.038). With a mass of ${\rm M}_{\star}$ = $1.284_{-0.016}^{+0.020}$ ${\rm M}_{J}$, a radius of ${\rm R}_{\star}$ = 2.314$_{-0.042}^{+0.049}$ ${\rm R_J}$, and an age of 4.7 $\pm$ 0.2 Gyr, the solar metallicity host star has already departed from the main sequence. We find evidence in the radial velocity measurements for a long term acceleration, and a $P \approx 18$ d periodic signal that we attribute to rotational modulation by stellar activity. The HD1397 system is among the brightest systems currently known to host a transiting planet, which will make it possible to perform detailed follow-up observations in order to characterize the properties of giant planets orbiting evolved stars.

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arXiv2018

EPIC 201498078b: A low density Super Neptune on an eccentric orbit

We report the discovery of EPIC 201498078b, which was first identified as a planetary candidate from Kepler K2 photometry of Campaign 14, and whose planetary nature and orbital parameters were then confirmed with precision radial velocities. EPIC 201498078b is half as massive as Saturn ($\rm M_P$=$0.179 \pm 0.021$ $\rm M_J$), and has a radius of $\rm R_P$=$0.840 \pm 0.011$ $\rm R_J$, which translates into a bulk density of $ \rm ρ_P$=$0.37 \pm 0.05$ g cm$^{-3}$. EPIC 201498078b transits its slightly evolved G-type host star ($\rm M_{\star}$=$1.105 \pm 0.019$ $\rm M_{\odot}$, $\rm R_P$=$1.669 \pm 0.022$ $\rm R_{\odot}$) every $11.63364 \pm 0.00010$ days and presents a significantly eccentric orbit ($e=0.420 \pm 0.034$). We estimate a relatively short circularization timescale of 1.8 Gyr for the planet, but given the advanced age of the system we expect the planet to be engulfed by its evolving host star in $\sim 1$ Gyr before the orbit circularizes. The low density of the planet coupled to the brightness of the host star ($J=9.4$) makes this system one of the best candidates known to date in the super-Neptune regime for atmospheric characterization via transmission spectroscopy, and to further study the transition region between ice and gas giant planets.

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arXiv2018

EPIC247098361b: a transiting warm Saturn on an eccentric $P=11.2$ days orbit around a $V=9.9$ star

We report the discovery of EPIC247098361b using photometric data of the Kepler K2 satellite coupled with ground-based spectroscopic observations. EPIC247098361b has a mass of M$_{P}=0.397\pm 0.037$ M$_J$, a radius of R$_{P}=1.00 \pm 0.020$ R$_J$, and a moderately low equilibrium temperature of $T_{eq}=1030 \pm 15$ K due to its relatively large star-planet separation of $a=0.1036$ AU. EPIC247098361b orbits its bright ($V=9.9$) late F-type host star in an eccentric orbit ($e=0.258 \pm 0.025$) every 11.2 days, and is one of only four well characterized warm Jupiters having hosts stars brighter than $V=10$. We estimate a heavy element content of 20 $\pm$ 7 M$_{\oplus}$ for EPIC247098361b, which is consistent with standard models of giant planet formation. The bright host star of EPIC247098361b makes this system a well suited target for detailed follow-up observations that will aid in the study of the atmospheres and orbital evolution of giant planets at moderate separations from their host stars.

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arXiv2017

K2-113b: A dense hot-Jupiter transiting a solar analogue

We present the discovery of K2-113b, a dense hot-Jupiter discovered using photometry from Campaign 8 of the Kepler-2 (K2) mission and high-resolution spectroscopic follow up obtained with the FEROS spectrograph. The planet orbits a $V=13.68$ solar analogue in a $P=5.81760^{+0.00003}_{-0.00003}$ day orbit, has a radius of $0.93^{+0.10}_{-0.07}R_J$ and a mass of $1.29^{+0.13}_{-0.14}M_J$. With a density of $1.97^{+0.60}_{-0.53}$ gr/cm$^3$, the planet is among the densest systems known having masses below 2 $M_J$ and $T_{eq} > 1000$, and is just above the temperature limit at which inflation mechanisms are believed to start being important. Based on its mass and radius, we estimate that K2-113b should have a heavy element content on the order of $\sim$ 110 $M_{\oplus}$ or greater.

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arXiv2017

HATS-22b, HATS-23b and HATS-24b: Three new transiting Super-Jupiters from the HATSouth Project

We report the discovery of three moderately high-mass transiting hot Jupiters from the HATSouth survey: HATS-22b, HATS-23b and HATS-24b. These planets add to the numbers of known planets in the ~2MJ regime. HATS-22b is a 2.74+/-0.11 MJ mass and 0.953+0.048/-0.029 RJ radius planet orbiting a V = 13.455 +/- 0.040 sub-silar mass (M_star = 0.759+/-0.019 M_sun; R_star = 0.759+/-0.019 R_sun) K-dwarf host star on an eccentric (e = 0.079 +/- 0.026) orbit. This planet's high planet-to-stellar mass ratio is further evidence that migration mechanisms for hot Jupiters may rely on exciting orbital eccentricities that bring planets closer to their parent stars followed by tidal circularisation. HATS-23b is a 1.478 +/- 0.080 MJ mass and 1.69 +/- 0.24 RJ radius planet on a grazing orbit around a V = 13.901 +/- 0.010 G-dwarf with properties very similar to those of the Sun (M_star = 1.115 +/- 0.054 M_sun; R_star = 1.145 +/- 0.070 R_sun). HATS-24b orbits a moderately bright V = 12.830 +/- 0.010 F-dwarf star (M_star = 1.218 +/- 0.036 M_sun; R_star = 1.194+0.066/-0.041 R_sun). This planet has a mass of 2.39 +0.21/-0.12 MJ and an inflated radius of 1.516 +0.085/-0.065 RJ.

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arXiv2016

HATS-18 b: An Extreme Short--Period Massive Transiting Planet Spinning Up Its Star

We report the discovery by the HATSouth network of HATS-18 b: a 1.980 +/- 0.077 Mj, 1.337 +0.102 -0.049 Rj planet in a 0.8378 day orbit, around a solar analog star (mass 1.037 +/- 0.047 Msun, and radius 1.020 +0.057 -0.031 Rsun) with V=14.067 +/- 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range 6.5 <= lg(Q*/k_2) <= 7 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star--planet interactions, be they gravitational, magnetic or radiative, as well as planet formation and migration theories.

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arXiv2016

Discovery and Validation of a High-Density sub-Neptune from the K2 Mission

We report the discovery of BD+20594b, a high density sub-Neptune exoplanet, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission, ground-based radial velocity follow-up from HARPS and high resolution lucky and adaptive optics imaging obtained using AstraLux and MagAO, respectively. The host star is a bright ($V=11.04$, $K_s = 9.37$), slightly metal poor ([Fe/H]$=-0.15\pm 0.05$ dex) solar analogue located at $152.1^{+9.7}_{-7.4}$ pc from Earth, for which we find a radius of $R_*=0.928^{+0.055}_{-0.040}R_\odot$ and a mass of $M_* = 0.961^{+0.032}_{-0.029}M_\odot$. A joint analysis of the K2 photometry and HARPS radial velocities reveal that the planet is in a $\approx 42$ day orbit around its host star, has a radius of $2.23^{+0.14}_{-0.11}R_\oplus$, and a mass of $16.3^{+6.0}_{-6.1}M_\oplus$. Although the data at hand puts the planet in the region of the mass-radius diagram where we could expect planets with a pure rock (i.e. magnesium silicate) composition using two-layer models (i.e., between rock/iron and rock/ice compositions), we discuss more realistic three-layer composition models which can explain the high density of the discovered exoplanet. The fact that the planet lies in the boundary between "possibly rocky" and "non-rocky" exoplanets, makes it an interesting planet for future RV follow-up.

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arXiv2016

An independent discovery of two hot Jupiters from the K2 mission

We report the discovery of two hot Jupiters using photometry from Campaigns 4 and 5 of the two-wheeled Kepler (K2) mission. K2-30b has a mass of $ 0.65 \pm 0.14 M_J$, a radius of $1.070 \pm 0.018 R_J$ and transits its G dwarf ($T_{eff} = 5675 \pm 50$ K), slightly metal rich ([Fe/H]$=+0.06\pm0.04$ dex) host star in a 4.1 days circular orbit. K2-34b has a mass of $ 1.63 \pm 0.12 M_J$, a radius of $1.38 \pm 0.014 R_J$ and has an orbital period of 3.0 days in which it orbits a late F dwarf ($T_{eff} = 6149 \pm 55$ K) solar metallicity star. Both planets were validated probabilistically and confirmed via precision radial velocity (RV) measurements. They have physical and orbital properties similar to the ones of the already uncovered population of hot Jupiters and are well-suited candidates for further orbital and atmospheric characterization via detailed follow-up observations. Given that the discovery of both systems was recently reported by other groups we take the opportunity of refining the planetary parameters by including the RVs obtained by these independent studies in our global analysis.

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arXiv2013

A Ground-based Optical Transmission Spectrum of WASP-6b

We present a ground based optical transmission spectrum of the inflated sub-Jupiter mass planet WASP-6b. The spectrum was measured in twenty spectral channels from 480 nm to 860nm using a series of 91 spectra over a complete transit event. The observations were carried out using multi-object differential spectrophotometry with the IMACS spectrograph on the Baade telescope at Las Campanas Observatory. We model systematic effects on the observed light curves using principal component analysis on the comparison stars, and allow for the presence of short and long memory correlation structure in our Monte Carlo Markov Chain analysis of the transit light curves for WASP-6. The measured transmission spectrum presents a general trend of decreasing apparent planetary size with wavelength and lacks evidence for broad spectral features of Na and K predicted by clear atmosphere models. The spectrum is consistent with that expected for scattering that is more efficient in the blue, as could be caused by hazes or condensates in the atmosphere of WASP-6b. WASP-6b therefore appears to be yet another massive exoplanet with evidence for a mostly featureless transmission spectrum, underscoring the importance that hazes and condensates can have in determining the transmission spectra of exoplanets.

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arXiv2011

A Search for the Transit of HD 168443b: Improved Orbital Parameters and Photometry

The discovery of transiting planets around bright stars holds the potential to greatly enhance our understanding of planetary atmospheres. In this work we present the search for transits of HD 168443b, a massive planet orbiting the bright star HD 168443 V=6.92 with a period of 58.11 days. The high eccentricity of the planetary orbit e=0.53 significantly enhances the a-priori transit probability beyond that expected for a circular orbit, making HD 168443 a candidate for our ongoing Transit Ephemeris Refinement and Monitoring Survey (TERMS). Using additional radial velocities from Keck-HIRES, we refined the orbital parameters of this multi-planet system and derived a new transit ephemeris for HD 168443b. The reduced uncertainties in the transit window make a photometric transit search practicable. Photometric observations acquired during predicted transit windows were obtained on three nights. CTIO 1.0 m photometry acquired on 2010 September 7 had the required precision to detect a transit but fell just outside of our final transit window. Nightly photometry from the T8 0.8 m Automated Photometric Telescope (APT) at Fairborn Observatory, acquired over a span of 109 nights, demonstrates that HD 168443 is constant on a time scale of weeks. Higher-cadence photometry on 2011 April 28 and June 25 shows no evidence of a transit. We are able to rule out a non-grazing transit of HD 168443b.

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arXiv2011

TERMS Photometry of Known Transiting Exoplanets

The Transit Ephemeris Refinement and Monitoring Survey (TERMS) conducts radial velocity and photometric monitoring of known exoplanets in order to refine planetary orbits and predictions of possible transit times. This effort is primarily directed towards planets not known to transit, but a small sample of our targets consist of known transiting systems. Here we present precision photometry for 6 WASP planets acquired during their transit windows. We perform a Markov Chain Monte Carlo (MCMC) analysis for each planet and combine these data with previous measurements to redetermine the period and ephemerides for these planets. These observations provide recent mid-transit times which are useful for scheduling future observations. Our results improve the ephemerides of WASP-4b, WASP-5b and WASP-6b and reduce the uncertainties on the mid-transit time for WASP-29b. We also confirm the orbital, stellar and planetary parameters of all 6 systems.

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