arXiv subjects

Gijs D. Mulders

Publications and source records attributed to Gijs D. Mulders.

At least 55 records · Page 3Linked to original sources

arXiv2018

Mid-infrared Multi-wavelength Imaging of Ophiuchus\,IRS\,48\,Transitional Disk

Transitional disks around the Herbig Ae/Be stars are fascinating targets in the contexts of disk evolution and also planet formation. Oph IRS 48 is one of such Herbig Ae stars, which shows an inner dust cavity and azimuthally lopsided large dust distribution. We present new images of Oph IRS 48 at eight mid-infrared (MIR) wavelengths from 8.59 to 24.6\,$μ{\rm m}$ taken with the COMICS mounted on the 8.2\,m Subaru Telescope. The N-band (7 to 13\,$μ{\rm m}$) images show that the flux distribution is centrally peaked with a slight spatial extent, while the Q-band (17 to 25\,$μ{\rm m}$) images show asymmetric double peaks (east and west). Using 18.8 and 24.6\,$μ$m images, we derived the dust temperature at both east and west peaks to be 135$\pm$22 K. Thus, the asymmetry may not be attributed to a difference in the temperature. % thus other reason is necessary to explain the asymmetry of east and west peaks in Q-band. Comparing our results with previous modeling works, we conclude that the inner disk is aligned to the outer disk. A shadow casted by the optically thick inner disk has a great influence on the morphology of MIR thermal emission from the outer disk.

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arXiv2018

A comprehensive understanding of planet formation is required for assessing planetary habitability and for the search for life

Dozens of habitable zone, approximately earth-sized exoplanets are known today. An emerging frontier of exoplanet studies is identifying which of these habitable zone, small planets are actually habitable (have all necessary conditions for life) and, of those, which are earth-like. Many parameters and processes influence habitability, ranging from the orbit through detailed composition including volatiles and organics, to the presence of geological activity and plate tectonics. While some properties will soon be directly observable, others cannot be probed by remote sensing for the foreseeable future. Thus, statistical understanding of planetary systems' formation and evolution is a key supplement to the direct measurements of planet properties. Probabilistically assessing parameters we cannot directly measure is essential to reliably assess habitability, to prioritizing habitable-zone planets for follow-up, and for interpreting possible biosignatures.

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arXiv2018

Exoplanet Diversity in the Era of Space-based Direct Imaging Missions

This whitepaper discusses the diversity of exoplanets that could be detected by future observations, so that comparative exoplanetology can be performed in the upcoming era of large space-based flagship missions. The primary focus will be on characterizing Earth-like worlds around Sun-like stars. However, we will also be able to characterize companion planets in the system simultaneously. This will not only provide a contextual picture with regards to our Solar system, but also presents a unique opportunity to observe size dependent planetary atmospheres at different orbital distances. We propose a preliminary scheme based on chemical behavior of gases and condensates in a planet's atmosphere that classifies them with respect to planetary radius and incident stellar flux.

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arXiv2018

A Universal Break in the Planet-to-Star Mass-Ratio Function of Kepler MKG stars

We follow the microlensing approach and quantify the occurrence of Kepler exoplanets as a function of planet-to-star mass ratio, q, rather than planet radius or mass. For planets with radii ~1-6 R_earth and periods <100 days, we find that, except for a normalization factor, the occurrence rate vs q can be described by the same broken power law with a break at ~3 x10^{-5} independent of host type for hosts below 1 M_sun. These findings indicate that the planet-to-star mass ratio is a more fundamental quantity in planet formation than planet mass. We then compare our results to those from microlensing for which the overwhelming majority satisfies the M_host<1 M_sun criterion. The break in q for the microlensing planet population, which mostly probes the region outside the snowline, is 3-10 times higher than that inferred from Kepler. Thus, the most common planet inside the snowline is ~3-10 times less massive than the one outside. With rocky planets interior to gaseous planets, the Solar System broadly follows the combined mass-ratio function inferred from Kepler and microlensing. However, the exoplanet population has a less extreme radial distribution of planetary masses than the Solar System. Establishing whether the mass-ratio function beyond the snowline is also host type independent will be crucial to build a comprehensive theory of planet formation.

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arXiv2018

Exoplanet Classification and Yield Estimates for Direct Imaging Missions

Future NASA concept missions that are currently under study, like Habitable Exoplanet Imaging Mission (HabEx) & Large Ultra-Violet Optical Infra Red (LUVOIR) Surveyor, would discover a large diversity of exoplanets. We propose here a classification scheme that distinguishes exoplanets into different categories based on their size and incident stellar flux, for the purpose of providing the expected number of exoplanets observed (yield) with direct imaging missions. The boundaries of this classification can be computed using the known chemical behavior of gases and condensates at different pressures and temperatures in a planetary atmosphere. In this study, we initially focus on condensation curves for sphalerite ZnS, H2O, CO2 and CH4. The order in which these species condense in a planetary atmosphere define the boundaries between different classes of planets. Broadly, the planets are divided into rocky (0.5 - 1.0RE), super-Earths (1.0- 1.75RE), sub-Neptunes (1.75-3.5RE), sub-Jovians (3.5 - 6.0RE) and Jovians (6-14.3RE) based on their planet sizes, and 'hot', 'warm' and 'cold' based on the incident stellar flux. We then calculate planet occurrence rates within these boundaries for different kinds of exoplanets, η_{planet}, using the community co-ordinated results of NASA's Exoplanet Program Analysis Group's Science Analysis Group-13 (SAG-13). These occurrence rate estimates are in turn used to estimate the expected exoplanet yields for direct imaging missions of different telescope diameter.

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arXiv2017

Constraints from Dust Mass and Mass Accretion Rate Measurements on Angular Momentum Transport in Protoplanetary Disks

We investigate the relation between disk mass and mass accretion rate to constrain the mechanism of angular momentum transport in protoplanetary disks. Dust mass and mass accretion rate in Chamaeleon I are correlated with a slope close to linear, similar to the one recently identified in Lupus. We investigate the effect of stellar mass and find that the intrinsic scatter around the best-fit Mdust-Mstar and Macc-Mstar relations is uncorrelated. Disks with a constant alpha viscosity can fit the observed relations between dust mass, mass accretion rate, and stellar mass, but over-predict the strength of the correlation between disk mass and mass accretion rate when using standard initial conditions. We find two possible solutions. 1) The observed scatter in Mdust and Macc is not primoridal, but arises from additional physical processes or uncertainties in estimating the disk gas mass. Most likely grain growth and radial drift affect the observable dust mass, while variability on large time scales affects the mass accretion rates. 2) The observed scatter is primordial, but disks have not evolved substantially at the age of Lupus and Chamaeleon I due to a low viscosity or a large initial disk radius. More accurate estimates of the disk mass and gas disk sizes in a large sample of protoplanetary disks, either through direct observations of the gas or spatially resolved multi-wavelength observations of the dust with ALMA, are needed to discriminate between both scenarios or to constrain alternative angular momentum transport mechanisms such as MHD disk winds.

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arXiv2017

An ALMA Survey of CO isotopologue emission from Protoplanetary Disks in Chamaeleon I

The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey $^{13}$CO and C$^{18}$O J = $3-2$ line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from 0.03 -- 2 M$_{\odot}$ in the nearby Chamaeleon I star-forming region. We detect $^{13}$CO emission from 17 sources and C$^{18}$O from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical ISM CO-to-H$_2$ ratios of $10^{-4}$, the resulting gas masses are implausibly low, with an average gas mass of $\sim$ 0.05 M$_{Jup}$ as inferred from the average flux of stacked $^{13}$CO lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.

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arXiv2017

Hints for Small Disks around Very Low-Mass Stars and Brown Dwarfs

The properties of disks around brown dwarfs and very-low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [OI] 63um line emission towards 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in a regime that connects the luminosity of our sources with brighter T~Tauri stars. We find VLMO disks with sizes [1.3--78] au, smaller than typical T~Tauri disks, fit well the spectral energy distributions assuming disk geometry and dust properties are stellar-mass independent. Reducing the disk size increases the disk temperature and we show that VLMOs do not follow previously derived disk temperature-stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [OI] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models we show that smaller disks can lead to the unexpected [OI] non-detections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA -- to establish if and how disk radii scale with stellar mass -- should be pursued further.

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arXiv2016

A Super-Solar Metallicity For Stars With Hot Rocky Exoplanets

The host star metallicity provide a measure of the conditions in protoplanetary disks at the time of planet formation. Using a sample of over 20,000 Kepler stars with spectroscopic metallicities from the LAMOST survey, we explore how the exoplanet population depends on host star metallicity as a function of orbital period and planet size. We find that exoplanets with orbital periods less than 10 days are preferentially found around metal-rich stars ([Fe/H]~ 0.15 +- 0.05 dex). The occurrence rates of these hot exoplanets increases to ~30% for super-solar metallicity stars from ~10% for stars with a sub-solar metallicity. Cooler exoplanets, that resides at longer orbital periods and constitute the bulk of the exoplanet population with an occurrence rate of >~ 90%, have host-star metallicities consistent with solar. At short orbital periods, P<10 days, the difference in host star metallicity is largest for hot rocky planets (<1.7 R_Earth), where the metallicity difference is [Fe/H] =~ 0.25 +- 0.07 dex. The excess of hot rocky planets around metal-rich stars implies they either share a formation mechanism with hot Jupiters, or trace a planet trap at the protoplanetary disk inner edge which is metallicity-dependent. We do not find statistically significant evidence for a previously identified trend that small planets toward the habitable zone are preferentially found around low-metallicity stars. Refuting or confirming this trend requires a larger sample of spectroscopic metallicities.

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arXiv2016

The Eruption of the Candidate Young Star ASASSN-15qi

Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The $\sim 3.5$ mag brightening in the $V$ band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from $\sim10,000$ K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km/s. The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitation of H$_2$ is detected in emission from vibrational levels as high as $v=11$, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, although the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.

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arXiv2016

The SPHERE view of the planet-forming disk around HD100546

We image with unprecedented spatial resolution and sensitivity disk features that could be potential signs of planet-disk interaction. Two companion candidates have been claimed in the disk around the young Herbig Ae/Be star HD100546. Thus, this object serves as an excellent target for our investigation of the natal environment of giant planets. We exploit the power of extreme adaptive optics operating in conjunction with the new high-contrast imager SPHERE to image HD100546 in scattered light. We obtain the first polarized light observations of this source in the visible (with resolution as fine as 2 AU) and new H and K band total intensity images that we analyze with the Pynpoint package. The disk shows a complex azimuthal morphology, where multiple scattering of photons most likely plays an important role. High brightness contrasts and arm-like structures are ubiquitous in the disk. A double-wing structure (partly due to ADI processing) resembles a morphology newly observed in inclined disks. Given the cavity size in the visible (11 AU), the CO emission associated to the planet candidate 'c' might arise from within the circumstellar disk. We find an extended emission in the K band at the expected location of 'b'. The surrounding large-scale region is the brightest in scattered light. There is no sign of any disk gap associated to 'b'.

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arXiv2015

An Increase in the Mass of Planetary Systems around Lower-Mass Stars

Trends in the planet population with host star mass provide an avenue to constrain planet formation theories. We derive the planet radius distribution function for Kepler stars of different spectral types, sampling a range in host star masses. We find that M dwarf stars have 3.5 times more small planets (1.0-2.8 R_Earth) than main-sequence FGK stars, but two times fewer Neptune-sized and larger planets (>2.8 R_Earth). We find no systematic trend in the planet size distribution between spectral types F, G, and K to explain the increasing occurrence rates. Taking into account the mass-radius relationship and heavy-element mass of observed exoplanets, and assuming those are independent of spectral type, we derive the inventory of the heavy-element mass locked up in exoplanets at short orbits. The overall higher planet occurrence rates around M stars are not consistent with the redistribution of the same mass into more, smaller planets. At the orbital periods and planet radii where Kepler observations are complete for all spectral types, the average heavy-element mass locked up in exoplanets increases roughly inversely with stellar mass from 4 M_Earth in F stars to 5 M_Earth in G and K stars to 7 M_Earth in M stars. This trend stands in stark contrast with observed protoplanetary disk masses that decrease towards lower mass stars, and provides a challenge for current planet formation models. Neither models of in situ formation nor migration of fully-formed planets are consistent with these results. Instead, these results are indicative of large-scale inward migration of planetary building blocks --- either through type-I migration or radial drift of dust grains --- that is more efficient for lower mass stars, but does not result in significantly larger or smaller planets.

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arXiv2015

Optical imaging polarimetry of the LkCa 15 protoplanetary disk with SPHERE ZIMPOL

We present the first optical (590--890 nm) imaging polarimetry observations of the pre-transitional protoplanetary disk around the young solar analog LkCa 15, addressing a number of open questions raised by previous studies. We detect the previously unseen far side of the disk gap, confirm the highly eccentric scattered-light gap shape that was postulated from near-infrared imaging, at odds with the symmetric gap inferred from millimeter interferometry. Furthermore, we resolve the inner disk for the first time and trace it out to 30 AU. This new source of scattered light may contribute to the near-infrared interferometric signal attributed to the protoplanet candidate LkCa 15 b, which lies embedded in the outer regions of the inner disk. Finally, we present a new model for the system architecture of LkCa 15 that ties these new findings together. These observations were taken during science verification of SPHERE ZIMPOL and demonstrate this facility's performance for faint guide stars under adverse observing conditions.

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arXiv2015

The snow line in viscous disks around low-mass stars: implications for water delivery to terrestrial planets in the habitable zone

The water ice or snow line is one of the key properties of protoplanetary disks that determines the water content of terrestrial planets in the habitable zone. Its location is determined by the properties of the star, the mass accretion rate through the disk, and the size distribution of dust suspended in the disk. We calculate the snow line location from recent observations of mass accretion rates and as a function of stellar mass. By taking the observed dispersion in mass accretion rates as a measure of the dispersion in initial disk mass, we find that stars of a given mass will exhibit a range of snow line locations. At a given age and stellar mass, the observed dispersion in mass accretion rates of 0.4 dex naturally leads to a dispersion in snow line locations of 0.2 dex. For ISM-like dust sizes, the one-sigma snow line location among solar mass stars of the same age ranges from 2 to 5 au. For more realistic dust opacities that include larger grains, the snow line is located up to two times closer to the star. We use these locations and the outcome of N-body simulations to predict the amount of water delivered to terrestrial planets that formed in situ in the habitable zone. We find that the dispersion in snow line locations leads to a large range in water content. For ISM-like dust sizes, a significant fraction of habitable-zone terrestrial planets around sun-like stars remain dry, and no water is delivered to the habitable zones of low-mass M stars (less than half a solar mass) as in previous works. The closer-in snow line in disks with larger grains enables water delivery to the habitable zone for a significant fraction of M stars and all FGK stars. Considering their larger numbers and higher planet occurrence, M stars may host most of the water-rich terrestrial planets in the galaxy if these planets are able to hold on to their water in their subsequent evolution.

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arXiv2015

Volatile Delivery to Planets from Water-rich Planetesimals around Low Mass Stars

Most models of volatile delivery to accreting terrestrial planets assume that the carriers for water are similar in water content to the carbonaceous chondrites in our Solar System. Here we suggest that the water content of primitive bodies in many planetary systems may actually be much higher, as carbonaceous chondrites have lost some of their original water due to heating from short-lived radioisotopes that drove parent body alteration. Using N-body simulations, we explore how planetary accretion would be different if bodies beyond the water line contained a water mass fraction consistent with chemical equilibrium calculations, and more similar to comets, as opposed to the more traditional water-depleted values. We apply this model to consider planet formation around stars of different masses and identify trends in the properties of Habitable Zone planets and planetary system architecture which could be tested by ongoing exoplanet census data collection. Comparison of such data with the model predicted trends will serve to evaluate how well the N-body simulations and the initial conditions used in studies of planetary accretion can be used to understand this stage of planet formation.

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arXiv2015

A stellar-mass-dependent drop in planet occurrence rates

The Kepler Spacecraft has discovered a large number of planets up to one-year periods and down to terrestrial sizes. While the majority of the target stars are main-sequence dwarfs of spectral type F, G, and K, Kepler covers stars with effective temperature as low as 2500 K, which corresponds to M stars. These cooler stars allow characterization of small planets near the habitable zone, yet it is not clear if this population is representative of that around FGK stars. In this paper, we calculate the occurrence of planets around stars of different spectral types as a function of planet radius and distance from the star, and show that they are significantly different from each other. We further identify two trends: First, the occurrence of Earth to Neptune-sized planets is successively higher toward later spectral types at all orbital periods probed by Kepler; Planets around M stars occur twice as frequently as around G stars, and thrice as frequently as around F stars. Second, a drop in planet occurrence is evident at all spectral types inward of a 10 day orbital period, with a plateau further out. By assigning to each spectral type a median stellar mass, we show that the distance from the star where this drop occurs is stellar mass dependent, and scales with semi-major axis as the cube root of stellar mass. By comparing different mechanisms of planet formation, trapping and destruction, we find that this scaling best matches the location of the pre-main-sequence co-rotation radius, indicating efficient trapping of migrating planets or planetary building blocks. These results demonstrate the stellar-mass dependence of the planet population, both in terms of occurrence rate and of orbital distribution. The prominent stellar-mass dependence of the inner boundary of the planet population shows that the formation or migration of planets is sensitive to the stellar parameters.

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arXiv2014

Resolved images of the protoplanetary disk around HD 100546 with ALMA

The disk around the Herbig Ae/Be star HD 100546 has been extensively studied and it is one of the systems for which there are observational indications of ongoing and/or recent planet formation. However, up until now no resolved image of the millimeter dust emission or the gas has been published. We present the first resolved images of the disk around HD 100546 obtained in Band 7 with the ALMA observatory. The CO (3-2) image reveals a gas disk that extends out to 350 au radius at the 3-sigma level. Surprisingly, the 870um dust continuum emission is compact (radius <60 au) and asymmetric. The dust emission is well matched by a truncated disk with outer radius of $\approx$50 au. The lack of millimeter-sized particles outside the 60 au is consistent with radial drift of particles of this size. The protoplanet candidate, identified in previous high-contrast NACO/VLT L' observations, could be related to the sharp outer edge of the millimeter-sized particles. Future higher angular resolution ALMA observations are needed to determine the detailed properties of the millimeter emission and the gas kinematics in the inner region (<2arcsec). Such observations could also reveal the presence of a planet through the detection of circumplanetary disk material.

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arXiv2013

Planet or Brown Dwarf? Inferring the Companion Mass in HD 100546 from the Wall Shape using Mid-Infrared Interferometry

Giant planets form in protoplanetary disks while these disks are still gas-rich, and can reveal their presence through the annular gaps they carve out. HD 100546 is a gas-rich disk with a wide gap between between a radius of ~1 and 13 AU, possibly cleared out by a planetary companion or planetary system. We want to identify the nature of the unseen companion near the far end of the disk gap. We use mid-infrared interferometry at multiple baselines to constrain the curvature of the disk wall at the far end of the gap. We use 2D hydrodynamical simulations of embedded planets and brown dwarfs to estimate viscosity of the disk and the mass of a companion close to the disk wall. We find that the disk wall at the far end of the gap is not vertical, but rounded-off by a gradient in the surface density. Such a gradient can be reproduced in hydrodynamical simulations with a single, heavy companion (>=30...80 M_Jup) while the disk has viscosity of at least α >~ 5 10^-3. Taking into account the changes in the temperature structure after gap opening reduces the lower limit on the planet mass and disk viscosity to 20 M_Jup and α = 2 10^-3. The object in the disk gap of HD 100546 that shapes the disk wall is most likely a 60(+20)(-40) M_Jup brown dwarf, while the disk viscosity is estimated to be at least α = 2 10^-3. The disk viscosity is an important factor in estimating planetary masses from disk morphologies: more viscous disks need heavier planets to open an equally deep gap.

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