arXiv subjects

L. Ilsedore Cleeves

Publications and source records attributed to L. Ilsedore Cleeves.

At least 91 records · Page 5Linked to original sources

arXiv2021

Dynamical Masses and Stellar Evolutionary Model Predictions of M-Stars

In this era of Gaia and ALMA, dynamical stellar mass measurements provide benchmarks that are independent of observations of stellar characteristics and their uncertainties. These benchmarks can then be used to validate and improve stellar evolutionary models, which can lead to both imprecise and inaccurate mass predictions for pre-main-sequence, low-mass stars. We present the dynamical stellar masses derived from disks around three M-stars (FP Tau, J0432+1827, and J1100-7619) using ALMA observations of $^{12}$CO (J=2--1) and $^{13}$CO (J=2--1) emission. These are the first dynamical stellar mass measurements for J0432+1827 and J1100-7619 and the most precise measurement for FP Tau. Fiducial stellar evolutionary model tracks, which do not include any treatment of magnetic activity, agree with the dynamical measurement of J0432+1827 but underpredict the mass by $\sim$60\% for FP Tau and $\sim$80\% for J1100-7619. Possible explanations for the underpredictions include inaccurate assumptions of stellar effective temperature, undetected binarity for J1100-7619, and that fiducial stellar evolutionary models are not complex enough to represent these stars. In the former case, the stellar effective temperatures would need to be increased by $\sim$40K to $\sim$340K to reconcile the fiducial model predictions with the dynamically-measured masses. In the latter case, we show that the dynamical masses can be reproduced using results from stellar evolutionary models with starspots, which incorporate fractional starspot coverage to represent the manifestation of magnetic activity. Folding in low-mass M-stars from the literature and assuming that the stellar effective temperatures are imprecise but accurate, we find tentative evidence of a relationship between fractional starspot coverage and observed effective temperature for these young, cool stars.

astro-ph.SR↗

arXiv2020

The TW Hya Rosetta Stone Project I: Radial and vertical distributions of DCN and DCO+

Molecular D/H ratios are frequently used to probe the chemical past of Solar System volatiles. Yet it is unclear which parts of the Solar Nebula hosted an active deuterium fractionation chemistry. To address this question, we present 0".2-0".4 ALMA observations of DCO+ and DCN 2-1, 3-2 and 4-3 towards the nearby protoplanetary disk around TW Hya, taken as part of the TW Hya Rosetta Stone project, augmented with archival data. DCO+ is characterized by an excitation temperature of ~40 K across the 70 au radius pebble disk, indicative of emission from a warm, elevated molecular layer. Tentatively, DCN is present at even higher temperatures. Both DCO+ and DCN present substantial emission cavities in the inner disk, while in the outer disk the DCO+ and DCN morphologies diverge: most DCN emission originates from a narrow ring peaking around 30~au, with some additional diffuse DCN emission present at larger radii, while DCO+ is present in a broad structured ring that extends past the pebble disk. Based on parametric disk abundance models, these emission patterns can be explained by a near-constant DCN abundance exterior to the cavity, and an increasing DCO+ abundance with radius. There appears to be an active deuterium fractionation chemistry in multiple disk regions around TW Hya, but not in the cold planetesimal-forming midplane and in the inner disk. More observations are needed to explore whether deuterium fractionation is actually absent in these latter regions, and if its absence is a common feature, or something peculiar to the old TW Hya disk.

astro-ph.EP↗

arXiv2020

The TW Hya Rosetta Stone Project II: Spatially resolved emission of formaldehyde hints at low-temperature gas-phase formation

Formaldehyde (H$_2$CO) is an important precursor to organics like methanol (CH$_3$OH). It is important to understand the conditions that produce H$_2$CO and prebiotic molecules during star and planet formation. H$_2$CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold ($\lesssim 20$ K) dust grains. To understand which pathway dominates, gaseous H$_2$CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating "warm" conditions and $<3$ linked to cold formation in the solid-state. We present spatially resolved ALMA observations of multiple ortho- and para-H$_2$CO transitions in the TW Hya protoplanetary disk to test H$_2$CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of $33\pm2$ K, ($1.1\pm0.1)\times10^{12}$ cm$^{-2}$ and $25\pm2$ K, $(4.4\pm0.3)\times10^{11}$ cm$^{-2}$ for ortho- and para-H$_2$CO, respectively, and an OPR of $2.49\pm0.23$. A radially resolved analysis shows that the observed H$_2$CO emits mostly at rotational temperatures of 30-40 K, corresponding to a layer with $z/R\ge0.25$. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to $2.0\pm0.5$. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, lead us to speculate that gas-phase formation is responsible for the observed H$_2$CO across the TW Hya disk.

astro-ph.SR↗

arXiv2020

A case of simultaneous star and planet formation

While it is widely accepted that planets are formed in protoplanetary disks, there is still much debate on when this process happens. In a few cases protoplanets have been directly imaged, but for the vast majority of systems, disk gaps and cavities -- seen especially in dust continuum observations -- have been the strongest evidence of recent or on-going planet formation. We present ALMA observations of a nearly edge-on ($i = 75^{\circ}$) disk containing a giant gap seen in dust but not in $^{12}$CO gas. Inside the gap, the molecular gas has a warm (100 K) component coinciding in position with a tentative free-free emission excess observed with the VLA. Using 1D hydrodynamic models, we find the structure of the gap is consistent with being carved by a planet with 4-70 $M_{\rm Jup}$. The coincidence of free-free emission inside the planet-carved gap points to the planet being very young and/or still accreting. In addition, the $^{12}$CO observations reveal low-velocity large scale filaments aligned with the disk major axis and velocity coherent with the disk gas that we interpret as ongoing gas infall from the local ISM. This system appears to be an interesting case where both a star (from the environment and the disk) and a planet (from the disk) are growing in tandem.

astro-ph.EP↗

arXiv2020

Untangling Magnetic Complexity in Protoplanetary Disks with the Zeeman Effect

With the recent advent of circular polarization capabilities at the Atacama Large Millimeter/submillimeter Array (ALMA), Zeeman effect measurements of spectral lines are now possible as a means to directly probe line-of-sight magnetic fields in protoplanetary disks (PPDs). We present a modeling study that aims to guide physical interpretation of these anticipated observations. Using a fiducial density structure based on a typical ringed disk, we simulate line emission for the hyperfine components of the CN $J = 1-0$ transition with the POLARIS radiative transfer code. Since the expected magnetic field and typical CN distribution in PPDs remain largely unconstrained, we produce models with several different configurations. Corresponding integrated Stokes $I$ and $V$ profiles and 0.4 km/s resolution, 1'' beam convolved channel maps are presented. We demonstrate that the emission signatures from toroidally dominated magnetic fields are distinguishable from vertically dominated magnetic field based on channel map morphology. Due to line-of-sight and beam cancellation effects, disks with toroidal $\boldsymbol{B}$-field configurations result in significantly diminished Stokes $V$ emission. Complex magnetic fields therefore render the traditionally used method for inferring line-of-sight magnetic field strengths (i.e., fitting the derivative of the Stokes $I$ to the Stokes $V$ profile) ambiguous, since a given intrinsic field strength can yield a variety of Stokes $V$ amplitudes depending on the magnetic field geometry. In addition, gas gaps can create structure in the integrated Stokes $V$ profile that might mimic magnetic substructure. This method should therefore be applied with caution in PPD environments, and can only confidently be used as a measure of magnetic field strength if the disk's magnetic field configuration is well understood.

astro-ph.SR↗

arXiv2020

An unbiased ALMA spectral survey of the LkCa 15 and MWC 480 protoplanetary disks

The volatile contents of protoplanetary disks both set the potential for planetary chemistry and provide valuable probes of defining disk system characteristics such as stellar mass, gas mass, ionization, and temperature structure. Current disk molecular inventories are fragmented, however, giving an incomplete picture: unbiased spectral line surveys are needed to assess the volatile content. We present here an overview of such a survey of the protoplanetary disks around the Herbig Ae star MWC 480 and the T Tauri star LkCa 15 in ALMA Band 7, spanning $\sim$36 GHz from 275--317 GHz and representing an order of magnitude increase in sensitivity over previous single-dish surveys. We detect 14 molecular species (including isotopologues), with 5 species (C$^{34}$S, $^{13}$CS, H$_{2}$CS, DNC, and C$_2$D) detected for the first time in protoplanetary disks. Significant differences are observed in the molecular inventories of MWC~480 and LkCa~15, and we discuss how these results may be interpreted in light of the different physical conditions of these two disk systems.

astro-ph.SR↗

arXiv2020

An evolutionary study of volatile chemistry in protoplanetary disks

The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion of CO in the first ~0.5-1 Myr of the disk lifetime. We do not see evidence that C2H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C2H formation and HC15N fractionation) and sublimation of ices from infalling material (impacting HCN and C18O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime.

astro-ph.SR↗

arXiv2020

An ALMA Survey of H$_2$CO in Protoplanetary Disks

H$_2$CO is one of the most abundant organic molecules in protoplanetary disks and can serve as a precursor to more complex organic chemistry. We present an ALMA survey of H$_2$CO towards 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies. H$_2$CO is detected towards 13 disks and tentatively detected towards a 14th. We find both centrally-peaked and centrally-depressed emission morphologies, and half of the disks show ring-like structures at or beyond expected CO snowline locations. Together these morphologies suggest that H$_2$CO in disks is commonly produced through both gas-phase and CO-ice-regulated grain-surface chemistry. We extract disk-averaged and azimuthally-averaged H$_2$CO excitation temperatures and column densities for four disks with multiple H$_2$CO line detections. The temperatures are between 20-50K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H$_2$CO emission in disks is generally emerging from the warm molecular layer, with some contributions from the colder midplane. Applying the same H$_2$CO excitation temperatures to all disks in the survey, we find that H$_2$CO column densities span almost three orders of magnitude ($\sim 5 \times 10^{11} - 5 \times 10^{14} \mathrm{cm}^{-2}$). The column densities appear uncorrelated with disk size and stellar age, but Herbig Ae disks may have less H$_2$CO compared to T Tauri disks, possibly because of less CO freeze-out. More H$_2$CO observations towards Herbig Ae disks are needed to confirm this tentative trend, and to better constrain under which disk conditions H$_2$CO and other oxygen-bearing organics efficiently form during planet formation.

astro-ph.SR↗

arXiv2020

Chemistry Along Accretion Streams in a Viscously-Evolving Protoplanetary Disk

The composition of a protoplanetary disk is set by a combination of interstellar inheritance and gas and grain surface chemical reactions within the disk. The survival of inherited molecules, as well as the disk in situ chemistry depends on the local temperature, density and irradiation environment, which can change over time due to stellar and disk evolution, as well as transport in the disk. We address one aspect of this coupling between the physical and chemical evolution in disks by following accretion streamlines of gas and small grains in the disk midplane, while simultaneously taking the evolving star into account. This approach is computationally efficient and enables us to take into account changing physical conditions without reducing the chemical network. We find that many species are enhanced in the inner disk midplane in the dynamic model due to inward transport of cosmic-ray driven chemical products, resulting in, e.g., orders-of magnitude hydrocarbon enhancements at 1 au, compared to a static disk. For several other chemical families, there is no difference between the static and dynamic models, indicative of a robust chemical reset, while yet others show differences between static and dynamic models that depend on complex interactions between physics and chemistry during the inward track. The importance of coupling dynamics and chemistry when modeling the chemical evolution of protoplanetary disks is thus depends on what chemistry is of interest.

astro-ph.SR↗

arXiv2019

Modeling Time Dependent Water Chemistry Due to Powerful X-ray Flares from T-Tauri Stars

Young stars emit strong flares of X-ray radiation that penetrate the surface layers of their associated protoplanetary disks. It is still an open question as to whether flares create significant changes in disk chemical composition. We present models of the time-evolving chemistry of gas-phase water during X-ray flaring events. The chemistry is modeled at point locations in the disk between 1 and 50 au at vertical heights ranging from the mid-plane to the surface. We find that strong, rare flares, i.e., those that increase the unattenuated X-ray ionization rate by a factor of 100 every few years, can temporarily increase the gas-phase water abundance relative to H can by more than a factor of $\sim3-5$ along the disk surface (Z/R $\ge$ 0.3). We report that a "typical" flare, i.e., those that increase the unattenuated X-ray ionization rate by a factor of a few every few weeks, will not lead to significant, observable changes. Dissociative recombination of H$_3$O$^+$, water adsorption and desorption onto dust grains, and ultraviolet photolysis of water and related species are found to be the three dominant processes regulating the gas-phase water abundance. While the changes are found to be significant, we find that the effect on gas phase water abundances throughout the disk is short-lived (days). Even though we do not see a substantial increase in long term water (gas and ice) production, the flares' large effects may be detectable as time varying inner disk water 'bursts' at radii between 5 and 30 au with future far infrared observations.

astro-ph.SR↗

arXiv2019

Probing CO and N$_2$ Snow Surfaces in Protoplanetary Disks with N$_2$H$^+$ Emission

Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D extensions of snowlines in the outer disk regions, where radiative heating results in a decreasing temperature with disk height. CO and N$_2$ are two of the most abundant carriers of C, N and O. N$_2$H$^+$ can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N$_2$. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N$_2$H$^+$ at 0.2$''$-0.4$''$ resolution in the disks around LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive emission morphologies: N$_2$H$^+$ is either present in a bright, narrow ring surrounded by extended tenuous emission, or in a broad ring. These emission patterns can be explained by two different kinds of vertical temperature structures. Bright, narrow N$_2$H$^+$ rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N$_2$H$^+$ emission peaks between the CO and N$_2$ snowlines. Broad N$_2$H$^+$ rings come from disks with a thin VIRaM layer (V4046 Sgr, AS 209, IM Lup). We use a simple model to extract the first sets of CO and N$_2$ snowline pairs and corresponding freeze-out temperatures towards the disks with a thick VIRaM layer. The results reveal a range of N$_2$ and CO snowline radii towards stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks.

astro-ph.SR↗

arXiv2019

Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline

CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

astro-ph.EP↗

arXiv2019

Realizing the Unique Potential of ALMA to Probe the Gas Reservoir of Planet Formation

Understanding the origin of the astonishing diversity of exoplanets is a key question for the coming decades. ALMA has revolutionized our view of the dust emission from protoplanetary disks, demonstrating the prevalence of ring and spiral structures that are likely sculpted by young planets in formation. To detect kinematic signatures of these protoplanets and to probe the chemistry of their gas accretion reservoir will require the imaging of molecular spectral line emission at high angular and spectral resolution. However, the current sensitivity of ALMA limits these important spectral studies to only the nearest protoplanetary disks. Although some promising results are emerging, including the identification of the snowlines of a few key molecules and the first attempt at detecting a protoplanet's spiral wake, it is not yet possible to search for these important signatures in a population of disks in diverse environments and ages. Harnessing the tremendous power of (sub)mm observations to pinpoint and characterize the chemistry of planets in formation will require a major increase of ALMA's spectral sensitivity (5-10x), increase in instantaneous bandwidth (2x) at high spectral resolution, and improved angular resolution (2x) in the 2030 era.

astro-ph.SR↗

arXiv2019

The Disk Gas Mass and the Far-IR Revolution

The gaseous mass of protoplanetary disks is a fundamental quantity in planet formation. The presence of gas is necessary to assemble planetesimals, it determines timescales of giant planet birth, and it is an unknown factor for a wide range of properties of planet formation, from chemical abundances (X/H) to the mass efficiency of planet formation. The gas mass obtained from traditional tracers, such as dust thermal continuum and CO isotopologues, are now known to have significant (1 - 2 orders of magnitude) discrepancies. Emission from the isotopologue of H2, hydrogen deuteride (HD), offers an alternative measurement of the disk gas mass. Of all of the regions of the spectrum, the far-infrared stands out in that orders of magnitude gains in sensitivity can be gleaned by cooling a large aperture telescope to 8 K. Such a facility can open up a vast new area of the spectrum to exploration. One of the primary benefits of this far-infrared revolution would be the ability to survey hundreds of planet-forming disks in HD emission to derive their gaseous masses. For the first time, we will have statistics on the gas mass as a function of evolution, tracing birth to dispersal as a function of stellar spectral type. These measurements have broad implications for our understanding of the time scale during which gas is available to form giant planets, the dynamical evolution of the seeds of terrestrial worlds, and the resulting chemical composition of pre-planetary embryos carrying the elements needed for life. Measurements of the ground-state line of HD requires a space-based observatory operating in the far-infrared at 112 microns.

astro-ph.EP↗

arXiv2018

Science with an ngVLA: Organics in Disk Midplanes with the ngVLA

Planets assemble in the midplanes of protoplanetary disks. The compositions of dust and gas in the disk midplane region determine the compositions of nascent planets, including their chemical hospitality to life. In this context, the distributions of volatile organic material across the planet and comet forming zones is of special interest. These are difficult to access in the disk midplane at IR and even millimeter wavelengths due to dust opacity, which can veil the midplane, low intrinsic molecular abundances due to efficient freeze-out, and, in the case of mid-sized organics, a mismatch between expected excitation temperatures and accessible line upper energy levels. At ngVLA wavelengths, the dust is optically thin, enabling observations into the planet forming disk midplane. ngVLA also has the requisite sensitivity. Using TW Hya as a case study, we show that ngVLA will be able to map out the distributions of diagnostic organics, such as CH3CN, in nearby protoplanetary disks.

astro-ph.GA↗

arXiv2018

Science with an ngVLA: Observing the Effects of Chemistry on Exoplanets and Planet Formation

One of the primary mechanisms for inferring the dynamical history of planets in our Solar System and in exoplanetary systems is through observation of elemental ratios (i.e. C/O). The ability to effectively use these observations relies critically on a robust understanding of the chemistry and evolutionary history of the observed abundances. Significant efforts have been devoted to this area from within astrochemistry circles, and these efforts should be supported going forward by the larger exoplanetary science community. In addition, the construction of a next-generation radio interferometer will be required to test many of these predictive models in situ, while simultaneously providing the resolution necessary to pinpoint the location of planets in formation.

astro-ph.EP↗

arXiv2018

Constraining Gas-Phase Carbon, Oxygen, and Nitrogen in the IM Lup Protoplanetary Disk

We present new constraints on gas-phase C, N, and O abundances in the molecular layer of the IM Lup protoplanetary disk. Building on previous physical and chemical modeling of this disk, we use new ALMA observations of C$_2$H to constrain the C/O ratio in the molecular layer to be $\sim0.8$, i.e., higher than the solar value of $\sim0.54$. We use archival ALMA observations of HCN and H$^{13}$CN to show that no depletion of N is required (assuming an interstellar abundance of $7.5\times10^{-5}$ per H). These results suggest that an appreciable fraction of O is sequestered in water ice in large grains settled to the disk mid-plane. Similarly, a fraction of the available C is locked up in less volatile molecules. By contrast, N remains largely unprocessed, likely as N$_2$. This pattern of depletion suggests the presence of true abundance variations in this disk, and not a simple overall depletion of gas mass. If these results hold more generally, then combined CO, C$_2$H, and HCN observations of disks may provide a promising path for constraining gas-phase C/O and N/O during planet-formation. Together, these tracers offer the opportunity to link the volatile compositions of disks to the atmospheres of planets formed from them.

astro-ph.SR↗

arXiv2018

Chemistry During the Gas-rich Stage of Planet Formation

In this chapter we outline some of the basic understanding of the chemistry that accompanies planet formation. We discuss the basic physical environment which dictates the dominant chemical kinetic pathways for molecule formation. We focus on three zones from both observational and theoretical perspectives: (1) the planet forming midplane and ice/vapor transition zones (snow-lines), (2) the warm disk surface that is shielded from radiation, which can be readily accessed by todays observational facilities, and (3) the surface photodissociation layers where stellar radiation dominates. We end with a discussion of how chemistry influences planet formation along with how to probe the link between formation and ultimate atmospheric composition for gas giants and terrestrial worlds.

astro-ph.EP↗