arXiv subjects

Sean M. Andrews

Publications and source records attributed to Sean M. Andrews.

At least 37 records · Page 2Linked to original sources

arXiv2025

Hubble reveals complex multi-scale structure in the edge-on protoplanetary disk IRAS 23077+6707

We present high-resolution ($\lesssim 0.1''$) Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) imaging of the near edge-on ($i{\sim}80^\circ$) protoplanetary disk IRAS 23077+6707 ("Dracula's Chivito") obtained across six broadband filters spanning $0.4-1.6\,μ$m. These observations unveil the scattered light from this unusually large disk (${\sim}14''$, or ${\sim}4200\,\mathrm{au}$ at $300\,\mathrm{pc}$) in remarkable detail, revealing a rich tapestry of substructures, including brightness asymmetries and signatures of dynamical activity. Extended filaments are detected extending ${\sim}10''$ from the northern edges of both nebulae, while no comparable southern features are observed. In addition to large-scale asymmetries, the disk exhibits prominent wispy features that extend well above the midplane and are visible in all filters, suggesting a complex, possibly turbulent outer disk atmosphere shaped by infall, dynamical stirring, or gravitational instability. The central dark lane narrows from optical to near-IR wavelengths, and high-resolution millimeter data reveal compact midplane emission. Although our radiative transfer simulations show that the current data cannot yet distinguish between dust settling and no-settling scenarios, they underscore the need for resolved mid-infrared observations of this unique system. IRAS 23077+6707 thus represents a rare and valuable laboratory for studying the vertical structure, asymmetries, and evolutionary state of protoplanetary disks.

astro-ph.EP↗

arXiv2025

A Submillimeter Survey of CS Excitation in Protoplanetary Disks: Evidence of X-ray-Driven Sulfur Chemistry

The sulfur chemistry in protoplanetary disks influences the properties of nascent planets, including potential habitability. Although the inventory of sulfur molecules in disks has gradually increased over the last decade, CS is still the most commonly-observed sulfur-bearing species and it is expected to be the dominant gas-phase sulfur carrier beyond the water snowline. Despite this, few dedicated multi-line observations exist, and thus the typical disk CS chemistry is not well constrained. Moreover, it is unclear how that chemistry - and in turn, the bulk volatile sulfur reservoir - varies with stellar and disk properties. Here, we present the largest survey of CS to date, combining both new and archival observations from ALMA, SMA, and NOEMA of 12 planet-forming disks, covering a range of stellar spectral types and dust morphologies. Using these data, we derived disk-integrated CS gas excitation conditions in each source. Overall, CS chemistry appears similar across our sample with rotational temperatures of ${\approx}$10-40 K and column densities between 10$^{12}$-10$^{13}$ cm$^{-2}$. CS column densities do not show strong trends with most source properties, which broadly suggests that CS chemistry is not highly sensitive to disk structure or stellar characteristics. We do, however, identify a positive correlation between stellar X-ray luminosity and CS column density, which indicates that the dominant CS formation pathway is likely via ion-neutral reactions in the upper disk layers, where X-ray-enhanced S$^+$ and C$^+$ drive abundant CS production. Thus, using CS as a tracer of gas-phase sulfur abundance requires a nuanced approach that accounts for its emitting region and dependence on X-ray luminosity.

astro-ph.EP↗

arXiv2025

Measuring the Two-Dimensional Thermal Structures of Protoplanetary Disks

We present a flexible, annulus-by-annulus method to constrain the 2-D thermal structure of a protoplanetary disk from optically thick spectral line emission. Using synthetic disk models with a known temperature and density structure, we extracted the vertical emission surfaces and brightness temperatures in radial annuli for multiple CO isotopologue transitions and used them to infer the vertical temperature profiles. This approach reliably recovers the injected temperature structure despite noise and finite resolution. We demonstrated that even a modest set of emission lines can constrain the temperature across a wide range of radii and elevations. Nevertheless, biases in the extracted emission surfaces constitute a major source of systematic error. Finally, we applied this method to archival ALMA observations of the HD 163296 disk, revealing that simple parametric radial temperature models may obscure the complexity of real disks and that additional observations are necessary to distinguish between different models of the vertical structure. This flexible framework can be readily applied to other systems, helping to characterize the thermal environments that shape planet formation.

astro-ph.EP↗

arXiv2025

A Radially Resolved Magnetic Field Threading the Disk of TW Hya

We present a new approach to detecting and characterizing a magnetic field in protoplanetary disks through the differential broadening of unpolarized molecular emission from CN. To demonstrate this technique, we apply it to new ALMA observations of the full complement of hyperfine components from the $N=1-0$ transition, achieving a spatial and spectral resolution of ${\approx}\,0.5^{\prime\prime}$ and $80~{\rm m\,s^{-1}}$, respectively. By fitting a model that incorporates the velocity structure of the disk, the potential non-LTE excitation of the molecule, and the Zeeman effect, we recover a radially resolved magnetic field with a strength of ${\sim}10~{\rm mG}$ between 60 and 120~au. The morphology of the field is also inferred through azimuthal variations in the line broadening, revealing a predominantly poloidal field at 60~au, sharply transitioning to one within the disk plane outside of the gap at 82~au. The signal-to-noise ratio of the data meant that the planar component was unable to be decomposed into toroidal and radial components. Lower limits on the local gas density ($n({\rm H_2}) \gtrsim 10^8~{\rm cm^{-3}}$) from the excitation analysis of the CN emission correspond to a lower limit between 0.1 and 0.01 for the plasma $β$.

astro-ph.EP↗

arXiv2025

Detailed Microwave Continuum Spectra from Bright Protoplanetary Disks in Taurus

We present new observations that densely sample the microwave (4-360 GHz) continuum spectra from eight young systems in the Taurus region. Multi-component, empirical model prescriptions were used to disentangle the contributions from their dust disks and other emission mechanisms. We found partially optically thick, free-free emission in all these systems, with positive spectral indices (median $α_{\rm c} \approx 1$ at 10 GHz) and contributing 5-50% of the 43 GHz fluxes. There is no evidence for synchrotron or spinning dust grain emission contributions for these targets. The inferred dust disk spectra all show substantial curvature: their spectral indices decrease with frequency, from $α_{\rm d} \approx 2.8$-4.0 around 43 GHz to 1.7-2.1 around 340 GHz. This curvature suggests that a substantial fraction of the (sub)millimeter ($\gtrsim$ 200 GHz) dust emission may be optically thick, and therefore the traditional metrics for estimating dust masses are flawed. Assuming the emission at lower frequencies (43 GHz) is optically thin, the local spectral indices and fluxes were used to constrain the disk-averaged dust properties and estimate corresponding dust masses. These masses are roughly an order of magnitude higher ($\approx 1000 \, M_\oplus$) than those found from the traditional approach based on (sub)millimeter fluxes. These findings emphasize the value of broad spectral coverage - particularly extending to lower frequencies ($\sim$cm-band) - for accurately interpreting dust disk emission; such observations may help reshape our perspective on the available mass budgets for planet formation.

astro-ph.SR↗

arXiv2025

ALMA Reveals an Eccentricity Gradient in the Fomalhaut Debris Disk

We present evidence of a negative eccentricity gradient in the debris disk of the nearby A-type main sequence star, Fomalhaut. Fitting to the high-resolution, archival ALMA 1.32 mm continuum data for Fomalhaut (with a synthesised angular resolution of $0.76{\times}0.55''$; 4-6\,au), we present a model that describes the bulk properties of the disk (semi-major axis, width, and geometry) and its asymmetric morphology. The best-fit model incorporates a forced eccentricity gradient that varies with semi-major axis, $e_f\propto a^{n_\mathrm{pow}}$, a generalized form of the parametric models of Lynch & Lovell 2022, with $n_\mathrm{pow}{=}{-1.75}{\pm}0.16$. We show that this model is statistically preferred to models with constant forced and free eccentricities. In comparison to disk models with constant forced eccentricities, negative eccentricity gradient models broaden disk widths at pericenter versus apocenter, and increase disk surface densities at apocenter versus pericenter, both of which are seen in the Fomalhaut disk, and which we collectively term Eccentric Velocity Divergence. We propose single-planet architectures consistent with the model and investigate the stability of the disk over 440 Myr to planet-disk interactions via N-body modeling. We find that Fomalhaut's ring eccentricity plausibly formed during the protoplanetary disk stage, with subsequent planet-disk interactions responsible for carving the disk morphology.

astro-ph.EP↗

arXiv2025

Multi-Wavelength Dust Characterization of the HL Tau Disk and Implications for Planet Formation

We present a comprehensive analysis of the HL Tau dust disk by modeling its intensity profiles across six wavelengths (0.45 to 7.9 mm) with a resolution of 0.05 arcsec ($\sim7$ au). Using a Markov Chain Monte Carlo (MCMC) approach, we constrain key dust properties including temperature, surface density, maximum grain size, composition, filling factor, and size distribution. The full fitting, with all parameters free, shows a preference for organics-rich dust with a low filling factor in the outer region ($r \gtrsim 40$ au), where the spectral index is $\sim3.7$, but amorphous-carbon-rich dust also reasonably reproduces the observed intensity profiles. Considering the scattering polarization observed at 0.87 mm, compact, amorphous-carbon-rich dust is unlikely, and moderately porous dust is favored. Beyond 40 au, the maximum dust size is likely $\sim100~{\rm μm}$ if dust is compact or amorphous-carbon rich. However, if the dust is moderately porous and organics-rich, both the predicted dust surface density and dust size can be sufficiently large for the pebble accretion rate to reach $\sim10M_{\oplus}~{\rm Myr^{-1}}$ in most regions, suggesting that pebble accretion could be a key mechanism for forming planets in the disk. In contrast, if the dust is amorphous-carbon-rich, forming a giant planet core via pebble accretion is unlikely due to the combined effects of low dust surface density and small dust size required to match the observed emission, suggesting other mechanisms, such as disk fragmentation due to gravitational instability, may be responsible for planet formation in the HL Tau disk.

astro-ph.EP↗

arXiv2025

exoALMA. XVIII. Interpreting large scale kinematic structures as moderate warping

The exoALMA program gave an unprecedented view of the complex kinematics of protoplanetary disks, revealing diverse structures that remain poorly understood. We show that moderate disk warps ($\sim 0.5-2^\circ$) can naturally explain many of the observed large-scale velocity features with azimuthal wavenumber $m = 1$. Using a simple model, we interpret line-of-sight velocity variations as changes in the projected Keplerian rotation caused by warping of the disk. While not a unique explanation, this interpretation aligns with growing observational evidence that warps are common. We demonstrate that such warps can also produce spiral structures in scattered light and CO brightness temperature, with $\sim 10$ K variations in MWC 758. Within the exoALMA sample, warp properties correlate with stellar accretion rates, suggesting a link between the inner disc and outer disc kinematics. If warps cause large-scale kinematic structure, this has far reaching implications for turbulence, angular momentum transport, and planet formation.

astro-ph.EP↗

arXiv2025

Grand Design Spiral Arms in the Compact, Embedded Protoplanetary Disk of Haro 6-13

Millimeter continuum spiral arms have so far only been detected in a handful of protoplanetary disks, and thus we have a limited understanding of the circumstances in which they can form. In particular, substructures in small disks ($R\lessapprox 50$ au) have not been well-characterized in comparison with large disks. We present ALMA 1.3 mm continuum observations of the disk around the T Tauri star Haro 6-13 at a resolution of $\sim0.04''$ ($\sim5$ au). A pair of low-contrast spiral arms are detected at disk radii from $\sim10-35$ au. They can be approximated as Archimedean spirals with pitch angles ranging from $\sim10-30^\circ$. The low value of the disk-averaged spectral index between 1.3 and 3 mm ($α=2.1$) and the high brightness temperatures suggest that the millimeter continuum is likely optically thick and thus may hide sufficient mass for the disk to become gravitationally unstable and form spiral arms. CO observations have shown that Haro 6-13 is surrounded by an envelope, raising the possibility that infall is facilitating spiral arm formation.

astro-ph.EP↗

arXiv2025

SMA and NOEMA reveal asymmetric sub-structure in the protoplanetary disk of IRAS23077+6707

We present high-resolution data of IRAS 23077+6707 (`Dracula's Chivito') with the Submillimeter Array (SMA at 1.33 mm/225.5 GHz) and the Northern Extended Millimeter Array (NOEMA at 2.7 mm/111.7 GHz and 3.1 mm/96.2 GHz). IRAS 23077+6707 is a highly-inclined and newly discovered protoplanetary disk, first reported in 2024. We combine SMA baselines from the Compact, Extended and Very Extended arrays, and NOEMA baselines from its A and C configurations, and present continuum images with resolution ${\lesssim}0.8''$, which constitute the first sub-arcsecond resolution maps of IRAS 23077+6707. The images show extended linear emission that spans $5.6{-}6.1''$ as expected for a radially extended, highly-inclined protoplanetary disk. Accompanied with lower resolution data, we show that the disk has a steep spectral index, ranging from $α=3.2{-}3.9$. We present evidence of multiple radial emission peaks and troughs in emission, which may originate in disk rings and a central cavity. We further present evidence that these radial structures are asymmetric; hosting a a significant brightness asymmetry, with emission enhanced by up to 50% in the north versus the south. We discuss hypotheses about the potential origins of these features, including the possibility that IRAS 23077+6707 hosts a rare example of an eccentric protoplanetary disk, which can induce these radially asymmetric structures. We present a simple eccentric continuum model of IRAS 23077+6707, and show for an eccentricity of $e \approx 0.26$, that this can reproduce the bulk morphology of the emission.

astro-ph.EP↗

arXiv2025

The JDISC Survey: Linking the Physics and Chemistry of Inner and Outer Protoplanetary Disk Zones

Mid-infrared spectroscopy of protoplanetary disks provides a chemical inventory of gas within a few au, where planets are readily detected around older stars. With the JWST Disk Infrared Spectral Chemistry Survey (JDISCS), we explore demographic trends among 31 disks observed with MIRI (MRS) and with previous ALMA millimeter continuum imaging at high angular resolution (5-10 au). With these S/N $\sim$200-450 spectra, we report emission from H$_2$O, OH, CO, C$_2$H$_2$, HCN, CO$_2$, [Ne II], [Ne III], and [Ar II]. Emission from H$_2$O, OH and CO is nearly ubiquitous for low-mass stars, and detection rates of all molecules are higher than for similar disks observed with Spitzer-IRS. Slab model fits to the molecular emission lines demonstrate that emission from C$_2$H$_2$, HCN, and possibly CO$_2$ is optically thin; thus since column densities and emitting radii are degenerate, observations are actually sensitive to the total molecular mass. C$_2$H$_2$ and HCN emission also typically originate in a hotter region ($920^{+70}_{-130}$, $820^{+70}_{-130}$ K, respectively) than CO$_2$ ($600^{+200}_{-160}$ K). The HCN to cold H$_2$O luminosity ratios are generally smaller in smooth disks, consistent with more efficient water delivery via icy pebbles in the absence of large dust substructures. The molecular emission line luminosities are also correlated with mass accretion rates and infrared spectral indices, similar to trends reported from Spitzer-IRS surveys. This work demonstrates the power of combining multi-wavelength observations to explore inner disk chemistry as a function of outer disk and stellar properties, which will continue to grow as the sample of observed Class II systems expands in the coming JWST observation cycles.

astro-ph.SR↗

arXiv2025

exoALMA VII: Benchmarking Hydrodynamics and Radiative Transfer Codes

Forward modeling is often used to interpret substructures observed in protoplanetary disks. To ensure the robustness and consistency of the current forward modeling approach from the community, we conducted a systematic comparison of various hydrodynamics and radiative transfer codes. Using four grid-based hydrodynamics codes (FARGO3D, Idefix, Athena++, PLUTO) and a smoothed particle hydrodynamics code (Phantom), we simulated a protoplanetary disk with an embedded giant planet. We then used two radiative transfer codes (mcfost, RADMC-3D) to calculate disk temperatures and create synthetic 12CO cubes. Finally, we retrieved the location of the planet from the synthetic cubes using DISCMINER. We found strong consistency between the hydrodynamics codes, particularly in the density and velocity perturbations associated with planet-driven spirals. We also found a good agreement between the two radiative transfer codes: the disk temperature in mcfost and RADMC-3D models agrees within $\lesssim 3~\%$ everywhere in the domain. In synthetic $^{12}$CO channel maps, this results in brightness temperature differences within $\pm1.5$ K in all our models. This good agreement ensures consistent retrieval of planet's radial/azimuthal location with only a few % of scatter, with velocity perturbations varying $\lesssim 20~\%$ among the models. Notably, while the planet-opened gap is shallower in the Phantom simulation, we found that this does not impact the planet location retrieval. In summary, our results demonstrate that any combination of the tested hydrodynamics and radiative transfer codes can be used to reliably model and interpret planet-driven kinematic perturbations.

astro-ph.EP↗

arXiv2025

ExoALMA XIII. gas masses from N2H+ and C18O: a comparison of protoplanetary gas disk mass measurement techniques

The gas masses of protoplanetary disks are important but elusive quantities. In this work we present new ALMA observations of N2H+ (3-2) for 11 exoALMA disks. N2H+ is a molecule sensitive to CO freeze-out and has recently been shown to significantly improve the accuracy of gas masses estimated from CO line emission. We combine these new observations with archival N2H+ and CO isotopologue observations to measure gas masses for 19 disks, predominantly from the exoALMA Large program. For 15 of these disks the gas mass has also been measured using gas rotation curves. We show that the CO + N2H+ line emission-based gas masses typically agree with the kinematically measured ones within a factor 3 (1-2σ). Gas disk masses from CO + N2H+ are on average a factor 2.3(+0.7,-1.0) x lower than the kinematic disk masses, which could suggest slightly lower N2 abundances and/or lower midplane ionization rates than typically assumed. Herbig disks are found to have ISM level CO gas abundances based on their CO and N2H+ fluxes, which sets them apart from T-Tauri disks where abundances are typically 3-30x lower. The agreement between CO + N2H+ -based and kinematically measured gas masses is promising and shows that multi-molecule line fluxes are a robust tool to accurately measure disk masses at least for extended disks.

astro-ph.EP↗

arXiv2025

exoALMA XIV. Gas Surface Densities in the RX J1604.3-2130 A Disk from Pressure-broadened CO Line Wings

The gas surface density is one of the most relevant physical quantities in protoplanetary disks. However, its precise measurement remains highly challenging due to the lack of a direct tracer. In this study, we report the spatially-resolved detection of pressure-broadened line wings in the CO $J=3-2$ line in the RX J1604.3-2130 A transition disk as part of the exoALMA large program. Since pressure-broadened line wings are sensitive to the total gas volume density, we robustly constrain the radial dependence of the gas surface density and midplane pressure in the region located $50-110$ au from the central star, which encompasses the dust ring of the system. The peak radius of the midplane pressure profile matches the dust ring radial location, directly proving radial dust trapping at a gas pressure maximum. The peak gas surface density is $18-44\ {\rm g\ cm^{-2}}$} and decreases at radii interior to and exterior of the dust ring. A comparison of the gas and dust surface densities suggests that the disk turbulence is as low as $α_{\rm turb} \sim 2\times10^{-4}$. Despite dust trapping, the gas-to-dust surface density ratio at the ring peak is { $70-400$}, which implies already-formed protoplanets and/or less efficient dust trapping. The gas surface density drop at radii interior to the ring is consistent with a gas gap induced by a Jupiter-mass planet. The total gas mass within $50 < r < 110$ au is estimated to be $\sim 0.05-0.1\ M_\odot$ ($50-100\ {M_{\rm Jup}}$), suggesting that planetary system formation is possible.

astro-ph.EP↗

arXiv2025

exoALMA II: Data Calibration and Imaging Pipeline

The exoALMA Large Program was designed to search for subtle kinematic deviations from Keplerian motion, indicative of embedded planets, in high angular and spectral resolution Band 7 observations of $^{12}$CO, $^{13}$CO and CS emission from protoplanetary disks. This paper summarizes the calibration and imaging pipelines used by the exoALMA collaboration. With sources ranging in diameter from 2.4" to 13.8" when probed by $^{12}$CO, multiple antennae configurations were required to maximally recover all spatial information (including the ACA for 7 sources). Combining these datasets warranted particular care in their alignment during calibration and prior to imaging, so as not to introduce spurious features that might resemble the kinematic deviations being investigated. Phase decoherence was found in several datasets, which was corrected by an iterative self-calibration procedure, and we explored the effects of the order of operations of spatial alignment, flux scaling, and self-calibration. A number of different imaging sets were produced for the continuum and line emission, employing an iterative masking procedure that minimizes bias due to non-Keplerian motions in the disk.

astro-ph.EP↗

arXiv2025

exoALMA. VI. Rotating under Pressure: Rotation curves, azimuthal velocity substructures, and pressure variations

The bulk motion of the gas in protoplanetary disks around newborn stars is nearly Keplerian. By leveraging the high angular and spectral resolution of ALMA, we can detect small-scale velocity perturbations in molecular line observations caused by local gas pressure variations in the disk, possibly induced by embedded protoplanets. This paper presents the azimuthally averaged rotational velocity and its deviations from Keplerian rotation ($δ\upsilon_ϕ$) for the exoALMA sample, as measured in the $^{12}$CO and $^{13}$CO emission lines. The rotation signatures show evidence for vertically stratified disks, in which $^{13}$CO rotates faster than $^{12}$CO due to a distinct thermal gas pressure gradient at their emitting heights. We find $δ\upsilon_ϕ$-substructures in the sample on both small ($\sim$10 au) and large ($\sim$100 au) radial scales, reaching deviations up to 15% from background Keplerian velocity in the most extreme cases. More than 75% of the rings and 80% of the gaps in the dust continuum emission resolved in $δ\upsilon_ϕ$ are co-located with gas pressure maxima and minima, respectively. Additionally, gas pressure substructures are observed far beyond the dust continuum emission. For the first time, we determined the gas pressure derivative at the midplane from observations and found it to align well with the dust substructures within the given uncertainties. Based on our findings, we conclude that gas pressure variations are likely the dominant mechanism for ring and gap formation in the dust continuum.

astro-ph.EP↗

arXiv2025

exoALMA XV: Interpreting the height of CO emission layer

The availability of exquisite data and the development of new analysis techniques have enabled the study of emitting heights in proto-planetary disks. In this paper we introduce a simple model linking the emitting height of CO to the disk surface density and temperature structure. We then apply the model to measurements of the emitting height and disk temperature conducted as part of exoALMA, integrated with additional legacy measurements from the MAPS Large Programme, to derive CO column densities and surface density profiles (assuming a CO abundance) for a total of 14 disks. A unique feature of the method we introduce to measure surface densities is that it can be applied to optically thick observations, rather than optically thin as conventionally done. While we use our method on a sample of well studied disks where temperature structures have been derived using two emission lines, we show that reasonably accurate estimates can be obtained also when only one molecular transition is available. With our method we obtain independent constraints from $^{12}$CO and $^{13}$CO and we find they are in general good agreement using the standard $^{12}$C/$^{13}$C isotopic ratio. The masses derived from our method are systematically lower compared with the values derived dynamically from the rotation curve if using an ISM CO abundance, implying that CO is depleted by a median factor $\sim$20 with respect to the ISM value, in line with other works that find that CO is depleted in proto-planetary disks.

astro-ph.EP↗

arXiv2025

exoALMA V: Gaseous Emission Surfaces and Temperature Structures

Analysis of the gaseous component in protoplanetary disks can inform us about their thermal and physical structure, chemical composition, and kinematic properties, all of which are crucial for understanding various processes within the disks. By exploiting the asymmetry of the line emission, or via line profile analysis, we can locate the emitting surfaces. Here, we present the emission surfaces of the exoALMA sources in $^{12}$CO $J=3-2$, $^{13}$CO $J=3-2$, and CS $J=7-6$. We find that $^{12}$CO traces the upper disk atmosphere, with mean <$z/r$> values of $\approx$ 0.28, while $^{13}$CO and CS trace lower regions of the disk with mean values of $\approx$ 0.16 and $\approx$ 0.18, respectively. We find that $^{12}$CO <$z/r$> and the disk mass are positively correlated with each other; this relationship offers a straightforward way to infer the disk mass. We derive 2-D $r-z$ temperature distributions of the disks. Additionally, we search for substructure in the surfaces and radial intensity profiles; we find evidence of localized substructure in the emission surfaces and peak intensity profiles of nearly every disk, with this substructure often being co-incident between molecular tracers, intensity profiles, and kinematic perturbations. Four disks display evidence of potential photo-desorption, implying that this effect may be common even in low FUV star-forming regions. For most disks, we find that the physical and thermal structure is more complex than analytical models can account for, highlighting a need for more theoretical work and a better understanding of the role of projection effects on our observations.

astro-ph.EP↗