Debris Disks Research Articles

The heart of Sakurai’s object revealed by ALMA

We present high-angular-resolution observations of Sakurai’s object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU) reveals a bright compact central component whose spectral index indicates that it is composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations, and therefore our observations provide the first direct imaging of this disc. The H12CN(J = 4 → 3) line emission observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU) displays a bipolar structure with a north–south velocity gradient. From the position–velocity diagram of this emission, we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be i = 72°. The derived values for the de-projected expansion velocity and the radius of the disc are vexp = 53 km s−1 and R = 277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H12CN(J = 4 → 3) emission is of approximately 1000 km s−1. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60°. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai’s object during the 30 years following the occurrence of the born-again event, providing important constraints for future modelling efforts of this phenomenon.

Formation of Gaps in Self-gravitating Debris Disks by Secular Resonance in a Single-planet System. II. Toward a Self-consistent Model

High-resolution observations of several debris disks reveal structures such as gaps and spirals, suggestive of gravitational perturbations induced by underlying planets. Most existing studies of planet–debris disk interactions ignore the gravity of the disk, treating it as a reservoir of massless planetesimals. In this paper, we continue our investigation into the long-term interaction between a single eccentric planet and an external, massive debris disk. Building upon our previous work, here we consider not only the axisymmetric component of the disk’s gravitational potential, but also the nonaxisymmetric torque that the disk exerts on the planet (ignoring for now only the nonaxisymmetric component of the disk self-gravity). To this goal, we develop and test a semianalytic “N-ring” framework that is based on a generalized (softened) version of the classical Laplace–Lagrange secular theory. Using this tool, we demonstrate that even when the disk is less massive than the planet, not only can a secular resonance be established within the disk that leads to the formation of a wide gap, but that the very same resonance also damps the planetary eccentricity e p via a process known as resonant friction. The resulting gap is initially nonaxisymmetric (akin to those observed in HD 92945 and HD 206893), but evolves to become more axisymmetric (similar to that in HD 107146) as e p (t) → 0 with time. We also develop analytic understanding of these findings, finding good quantitative agreement with the outcomes of the N-ring calculations. Our results may be used to infer both the dynamical masses of (gapped) debris disks and the dynamical history of the planets interior to them, as we exemplify for HD 206893.

Isolating Dust and Free–Free Emission in ONC Proplyds with ALMA Band 3 Observations

The Orion Nebula Cluster (ONC) hosts protoplanetary disks experiencing external photoevaporation by the cluster’s intense UV field. These “proplyds” are comprised of a disk surrounded by an ionization front. We present ALMA Band 3 (3.1 mm) continuum observations of 12 proplyds. Thermal emission from the dust disks and free–free emission from the ionization fronts are both detected, and the high-resolution (0.″057) of the observations allows us to spatially isolate these two components. The morphology is unique compared to images at shorter (sub)millimeter wavelengths, which only detect the disks, and images at longer centimeter wavelengths, which only detect the ionization fronts. The disks are small (r d = 6.4–38 au), likely due to truncation by ongoing photoevaporation. They have low spectral indices (α ≲ 2.1) measured between Bands 7 and 3, suggesting the dust emission is optically thick. They harbor tens of Earth masses of dust as computed from the millimeter flux using the standard method although their true masses may be larger due to the high optical depth. We derive their photoevaporative mass-loss rates in two ways: first, by invoking ionization equilibrium and second, by using the brightness of the free–free emission to compute the density of the outflow. We find decent agreement between these measurements and = 0.6–18.4 × 10−7 M ⊙ yr−1. The photoevaporation timescales are generally shorter than the ∼1 Myr age of the ONC, underscoring the known “proplyd lifetime problem.” Disk masses that are underestimated due to being optically thick remains one explanation to ease this discrepancy.

Early Planet Formation in Embedded Disks (eDisk). V. Possible Annular Substructure in a Circumstellar Disk in the Ced110 IRS4 System

We have observed the Class 0/I protostellar system Ced110 IRS4 at an angular resolution of 0.″05 (∼10 au) as part of the Atacama Large Millimeter/submillimeter Array large program, Early Planet Formation in Embedded Disks. The 1.3 mm dust continuum emission reveals that Ced110 IRS4 is a binary system with a projected separation of ∼250 au. The continuum emissions associated with the main source and its companion, named Ced110 IRS4A and IRS4B, respectively, exhibit disk-like shapes and likely arise from dust disks around the protostars. The continuum emission of Ced110 IRS4A has a radius of ∼91.7 au (∼0.″485) and shows bumps along its major axis with an asymmetry. The bumps can be interpreted as a shallow, ring-like structure at a radius of ∼40 au (∼0.″2) in the continuum emission, as demonstrated from two-dimensional intensity distribution models. A rotation curve analysis on the C18O and 13CO J = 2–1 lines reveals the presence of a Keplerian disk within a radius of 120 au around Ced110 IRS4A, which supports the interpretation that the dust continuum emission arises from a disk. The ring-like structure in the dust continuum emission might indicate a possible annular substructure in the surface density of the embedded disk, although the possibility that it is an apparent structure due to the optically thick continuum emission cannot be ruled out.

Refining the Stellar Parameters of τ Ceti: a Pole-on Solar Analog

To accurately characterize the planets a star may be hosting, stellar parameters must first be well determined. τ Ceti is a nearby solar analog and often a target for exoplanet searches. Uncertainties in the observed rotational velocities have made constraining τ Ceti’s inclination difficult. For planet candidates from radial velocity (RV) observations, this leads to substantial uncertainties in the planetary masses, as only the minimum mass () can be constrained with RV. In this paper, we used new long-baseline optical interferometric data from the CHARA Array with the MIRC-X beam combiner and extreme precision spectroscopic data from the Lowell Discovery Telescope with EXPRES to improve constraints on the stellar parameters of τ Ceti. Additional archival data were obtained from a Tennessee State University Automatic Photometric Telescope and the Mount Wilson Observatory HK project. These new and archival data sets led to improved stellar parameter determinations, including a limb-darkened angular diameter of 2.019 ± 0.012 mas and rotation period of 46 ± 4 days. By combining parameters from our data sets, we obtained an estimate for the stellar inclination of 7° ± 7°. This nearly pole-on orientation has implications for the previously reported exoplanets. An analysis of the system dynamics suggests that the planetary architecture described by Feng et al. may not retain long-term stability for low orbital inclinations. Additionally, the inclination of τ Ceti reveals a misalignment between the inclinations of the stellar rotation axis and the previously measured debris disk rotation axis (i disk = 35° ± 10°).

Long-term Evolution of Warps in Debris Disks—Application to the Gyr-old System HD 202628

We present the results of N-body simulations meant to reproduce the long-term effects of mutually inclined exoplanets on debris disks, using the HD 202628 system as a proxy. HD 202628 is a Gyr-old solar-type star that possesses a directly observable, narrow debris ring with a clearly defined inner edge and nonzero eccentricity, hinting at the existence of a sculpting exoplanet. The eccentric nature of the disk leads us to examine the effect on it over Gyr timescales from an eccentric and inclined planet, placed on its orbit through scattering processes. We find that, in systems with dynamical timescales akin to that of HD 202628, a planetary companion is capable of completely tilting the debris disk. This tilt is preserved over the Gyr age of the system. Simulated observations of our models show that an exoplanet around HD 202628 with an inclination misalignment ≳10° would cause the disk to be observably diffuse and broad, which is inconsistent with Atacama Large Millimeter Array (ALMA) observations. With these observations, we conclude that, if there is an exoplanet shaping this disk, it likely had a mutual inclination of less than 5° with the primordial disk. The conclusions of this work can be applied either to debris disks appearing as narrow rings (e.g., Fomalhaut and HR 4796) or to disks that are vertically thick at ALMA wavelengths (e.g., HD 110058).

Evidence for Misalignment between Debris Disks and Their Host Stars

We place lower limits on the obliquities between debris disks and their host stars for 31 systems by comparing their disk and stellar inclinations. While previous studies did not find evidence for misalignment, we identify six systems with minimum obliquities falling between ∼30°and 60°, indicating that debris disks can be significantly misaligned with their stars. These high-obliquity systems span a wide range of stellar parameters with spectral types K through A. Previous works have argued that stars with masses below 1.2 M ⊙ (spectral types of ∼F6) have magnetic fields strong enough to realign their rotation axes with the surrounding disk via magnetic warping; given that we observe high obliquities for relatively low-mass stars, magnetic warping alone is likely not responsible for the observed misalignments. Yet, chaotic accretion is expected to result in misalignments of ∼20° at most and cannot explain the larger obliquities found in this work. While it remains unclear how primordial misalignment might occur and what role it plays in determining the spin–orbit alignment of planets, future work expanding this sample is critical toward understanding the mechanisms that shape these high-obliquity systems.

Searching for magnetic fields in pulsating A-type stars: the discovery of a strong field in the probable δ Sct star HD 340577 and a null result for the γ Dor star HR 8799

ABSTRACT Numerous δ Sct and γ Dor pulsators are identified in the region of the Hertzsprung–Russell diagram that is occupied by chemically peculiar magnetic Ap stars. The connection between δ Sct and γ Dor pulsations and the magnetic field in Ap stars is however not clear: theory suggests for magnetic Ap stars some critical field strengths for pulsation mode suppression by computing the magnetic damping effect for selected p and g modes. To test these theoretical considerations, we obtained PEPSI spectropolarimetric snapshots of the typical Ap star HD 340577, for which δ Sct-like pulsations were recently detected in Transiting Exoplanet Survey Satellite data, and the γ Dor pulsator HR 8799, which is a remarkable system with multiple planets and two debris discs. Our measurements reveal the presence of a magnetic field with a strength of several hundred Gauss in HD 340577. The measured mean longitudinal field would be the strongest field measured so far in a δ Sct star if the pulsational character of HD 340577 is confirmed spectroscopically. No magnetic field is detected in HR 8799.

On Secular Gravitational Instability in Vertically Stratified Disks

Secular gravitational instability (GI) is one promising mechanism for explaining planetesimal formation. Previous studies of secular GI utilized a razor-thin disk model and derived the growth condition in terms of vertically integrated physical values such as dust-to-gas surface density ratio. However, in weakly turbulent disks where secular GI can operate, a dust disk can be orders of magnitude thinner than a gas disk, and analyses treating the vertical structures are necessary to clarify the interplay of the midplane dust motion and the upper gas motion. In this work, we perform vertically global linear analyses of secular GI with a vertical domain size of a few gas scale heights. We find that dust grains accumulate radially around the midplane while gas circulates over the whole vertical region. We obtain well-converged growth rates when the outer gas boundary is above two gas scale heights. The growth rates are underestimated if we assume the upper gas to be steady and regard it just as the source of external pressure to the dusty lower layer. Therefore, treating the upper gas motion is important even when the dust disk is much thinner than the gas disk. Conducting a parameter survey, we represent the growth condition in terms of the Toomre Q value for dust and dust-to-gas surface density ratio. The critical dust disk mass for secular GI is ∼10−4 M * for a dust-to-gas surface density ratio of 0.01, a Stokes number of 0.1, and a radial dust diffusivity of 10−4 c s H, where c s is the gas sound speed, and H is the gas scale height.

Massive pre-main-sequence stars in M17

Context. Recently much progress has been made in probing the embedded stages of massive star formation, pointing to formation scenarios that are reminiscent of a scaled-up version of low-mass star formation. However, the latest stages of massive-star formation have rarely been observed, as young massive stars are assumed to reveal their photospheres only when they are fully formed. Aims. Using first and second overtone CO bandhead emission and near- to mid-infrared photometry, we aim to characterize the remnant formation disks around five unique pre-main-sequence (PMS) stars with masses 6–12 M⊙ that have constrained stellar parameters thanks to their detectable photospheres. We seek to understand this emission and the disks from which it originates in the context of the evolutionary stage of the studied sources. Methods. We used an analytic disk model, and adopted local thermodynamical equilibrium, to fit the CO bandhead and the dust emission, assumed to originate in different disk regions. For the first time, we modeled the second overtone emission, which helped us to put tighter constraints on the density of the CO gas. Furthermore, we fit continuum normalized bandheads, using models for stellar and dust continuum, and show the importance of this in constraining the emission region. We also included 13CO in our models as an additional probe of the young nature of the studied objects. Results. We find that the CO emission originates in a narrow region close to the star (<1 AU) and under very similar disk conditions (temperatures and densities) for the different objects. This is consistent with previous modeling of this emission in a diverse range of young stellar objects and identifies CO emission as an indicator of the presence of a gaseous inner disk reaching close to the stellar surface. From constraining the location of the inner edge of the dust emission, we find that all but one of the objects have undisrupted inner dust disks. Conclusions. We discuss these results in the context of the positions of these PMS stars in the Hertzsprung-Russel diagram and the CO emission’s association with an early age and high accretion rates in (massive) young stellar objects. We conclude, considering their mass range and the fact that their photospheres are detected, that the M17 PMS stars are observed in a relatively early formation stage. They are therefore excellent candidates for longer wavelength studies to further constrain the end stages of massive star formation.

Quantitative polarimetry for the transition disk in RX J1604.3-213010

Context. The characterization of the dust in protoplanetary disks is important for a better understanding of the resulting composition of forming planets and the dust particle evolution in these systems. Aims. We aim to accurately characterize the properties of the dust in the face-on transition disk around RX J1604.3–213010 (RX J1604) by analyzing the multiwavelength scattered light intensity and polarization images obtained with the ZIMPOL and IRDIS subinstruments of VLT/SPHERE. Methods. We used archival data of RX J1604 from the ESO archive and carefully corrected the polarization signal for instrumental effects, also taking the interstellar polarization into account. We measured the radial profiles of the disk for the azimuthal polarization, Qφ(r), in the R, J, and H bands and describe variations in our data due to the seeing and other effects. We derived the intrinsic polarization profiles of the disk, Q^φ(r), by comparing the data with rotationally symmetric models convolved with the point spread functions of the observations. We also measured the disk intensity, Idisk(r), with reference star differential imaging for the J and H bands. This provides the disk-integrated polarized intensity, Q^φ/I⋆, for the R, J, and H bands and the averaged fractional polarization, 〈p^φ〉, for the J and H bands. We investigated the azimuthal dependence of the scattered light and the shadows produced by hot dust near the star. The derived results were finally compared with model calculations to constrain the scattering properties of the reflecting dust in RX J1604. Results. RX J1604 is a dipper source, and the data show different kinds of variability. However, a detailed analysis of repeated measurements shows that the results are not affected by dipping events or atmospheric seeing variations. We derive accurate radial disk profiles for the intrinsic polarized intensity, Q^φ(r)/I⋆, and measure different profile peak radii for different bands because of the wavelength dependence of the dust opacity. The disk-integrated polarization is Q^φ/I⋆ = 0.92 ± 0.04% for the R band and 1.51 ± 0.11% for the J band, indicating a red color for the polarized reflectivity of the disk. The intensity of the disk is Idisk|I* = 3.9 ± 0.5% in the J band, and the fractional polarization is 〈p^φ〉 = 38 ± 4% for the J band and 42 ± 2% for the H band. The comparison with the IR excess for RX J1604 yields an apparent disk albedo of about ΛΙ ≈ 0.16 ± 0.08. We also find that previously described shadows seen in the R band data are likely affected by calibration errors. We derive, using dust scattering models for transition disks, approximate J band values for the scattering albedo ω ≈ 0.5, scattering asymmetry g ≈ 0.5, and scattering polarization pmax ≈ 0.7 for the dust. Conclusions. The bright disk of RX J1604 has a very simple axisymmetric structure and is therefore well suited as a benchmark object for accurate photo-polarimetric measurements. We derive values for the disk polarization, 〈p^φ〉, and the apparent disk albedo, ΛΙ, for the J band. Because 〈p^φ〉 and ΛΙ depend predominantly on dust scattering parameters and only weakly on the disk geometry, these parameters define tight relations for the dust scattering parameters between ω and pmax and between ω and g. The positive R to J band color for the polarized reflectivity, (Q^/I⋆)J ≈ 1.64 ⋅ (Q^/I⋆)R, is mainly a result of the wavelength dependence of dust parameters because the scattering geometry is expected to be very similar for different colors. This work demonstrates the potential of accurate photo-polarimetric measurements of the circumstellar disk RX J1604 for the determination of dust scattering parameters that strongly constrain the physical properties of the dust.

Substructures in Compact Disks of the Taurus Star-forming Region

Observations of substructure in protoplanetary disks have largely been limited to the brightest and largest disks, excluding the abundant population of compact disks, which are likely sites of planet formation. Here, we reanalyze ∼0.″1, 1.33 mm Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations of 12 compact protoplanetary disks in the Taurus star-forming region. By fitting visibilities directly, we identify substructures in six of the 12 compact disks. We then compare the substructures identified in the full Taurus sample of 24 disks in single-star systems and the ALMA DSHARP survey, differentiating between compact (R eff,90% < 50 au) and extended (R eff,90% ≥50 au) disk sources. We find that substructures are detected at nearly all radii in both small and large disks. Tentatively, we find fewer wide gaps in intermediate-sized disks with R eff,90% between 30 and 90 au. We perform a series of planet–disk interaction simulations to constrain the sensitivity of our visibility-fitting approach. Under the assumption of planet–disk interaction, we use the gap widths and common disk parameters to calculate potential planet masses within the Taurus sample. We find that the young planet occurrence rate peaks near Neptune masses, similar to the DSHARP sample. For 0.01 M J/M ⊙ ≲M p/M * ≲0.1 M J/M ⊙, the rate is 17.4% ± 8.3%; for 0.1 M J/M ⊙ ≲M p/M * ≲1 M J/M ⊙, it is 27.8% ± 8.3%. Both of them are consistent with microlensing surveys. For gas giants more massive than 5 M J, the occurrence rate is 4.2% ± 4.2%, consistent with direct imaging surveys.

Reinterpreting the Polluted White Dwarf SDSS J122859.93+104032.9 in Light of Thermohaline Mixing Models: More Polluting Material from a Larger Orbiting Solid Body

The polluted white dwarf (WD) system SDSS J122859.93+104032.9 (SDSS J1228) shows variable emission features interpreted as originating from a solid core fragment held together against tidal forces by its own internal strength, orbiting within its surrounding debris disk. Estimating the size of this orbiting solid body requires modeling the accretion rate of the polluting material that is observed mixing into the WD surface. That material is supplied via sublimation from the surface of the orbiting solid body. The sublimation rate can be estimated as a simple function of the surface area of the solid body and the incident flux from the nearby hot WD. On the other hand, estimating the accretion rate requires detailed modeling of the surface structure and mixing in the accreting WD. In this work, we present MESA WD models for SDSS J1228 that account for the thermohaline instability and mixing in addition to heavy element sedimentation to constrain accurately the sublimation and accretion rate necessary to supply the observed pollution. We derive a total accretion rate of , several orders of magnitude higher than the estimate obtained in earlier efforts. The larger mass accretion rate implies that the minimum estimated radius of the orbiting solid body is = 72 km, which, although significantly larger than prior estimates, still lies within the upper bounds (a few hundred kilometers) for which the internal strength could no longer withstand the tidal forces from the gravity of the WD.

The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey

The spatial distribution and evolution of gas in the inner 10 au of protoplanetary disks form the basis for estimating the initial conditions of planet formation. Among the most important constraints derived from spectroscopic observations of the inner disk are the radial distributions of the major gas phase constituents, how the properties of the gas change with inner disk dust evolution, and how the chemical abundances and excitation conditions are influenced by the high-energy radiation from the central star. We present a survey of the radial distribution, excitation, and evolution of inner disk molecular hydrogen (H2) obtained as part of the Hubble Space Telescope-ULLYSES program. We analyze far-UV spectroscopy of 71 (63 accreting) pre-main-sequence systems in ULLYSES DR5 to characterize the H2 emission lines, H2 dissociation continuum emission, and major photochemical/disk evolution driving the UV emissions (Lyα, UV continuum, and C iv). We use the widths of the H2 emission lines to show that most fluorescent H2 arises between 0.1 and 1.4 au from the parent star, and show positive correlations of the average emitting radius with the accretion luminosity and with the dust disk mass. We find a strong correlation between H2 dissociation emission and both the accretion-dominated Lyα luminosity and the inner disk dust clearing, painting a picture where water molecules in the inner 3 au are exposed to and dissociated by strong Lyα emission as the opacity of the inner disk declines with time.

ALMA and Keck analysis of Fomalhaut field sources: JWST’s Great Dust Cloud is a background object

ABSTRACT At 7.7 pc, the A-type star Fomalhaut hosts a bright debris disc with multiple radial components. The disc is eccentric and misaligned, strongly suggesting that it is sculpted by interaction with one or more planets. Compact sources are now being detected with JWST, suggesting that new planet detections may be imminent. However, to confirm such sources as companions, common proper motion with the star must be established, as with unprecedented sensitivity comes a high probability that planet candidates are actually background objects. Here, Atacama Large Millimetre/Submillimetre Array and Keck observations of Fomalhaut are found to show significant emission at the same sky location as multiple compact sources in JWST Mid-Infrared Instrument coronagraphic observations, one of which has been dubbed the ‘Great Dust Cloud’ because it lies within the outer belt. Since the ground-based data were obtained between 6 and 18 yr prior to the JWST observations, these compact sources are unlikely to be common proper motion companions to Fomalhaut. More generally, this work illustrates that images collected at a range of wavelengths can be valuable for rejecting planet candidates uncovered via direct imaging with JWST.

Dynamical Stability of Polar Circumbinary Orbits and Planet Formation in the Planetary Disk of 99 Herculis

A possible polar-ring debris disk, the dynamics of which can be described by the outer hierarchical restricted three-body problem, has been detected in 99 Herculis. An empirical formula for the minimum radius beyond which test particles in polar orbits can keep stable within 107 binary periods is provided through numerical fitting, applying to the binary eccentricity and the mass ratio of the binary , where λ = m 0/m 1 (m 0 and m 1 represent the masses of the two binary stars). The polar planetary disk has the lowest statistical accretion efficiency and a moderate impact frequency of collisions among planetesimals (with radii of 1–10 km) compared to those in a circumbinary coplanar disk and a standard disk around the single host star. The collision timescale in the circumbinary disk (both polar and coplanar configurations) is longer than 107 yr, exceeding the dissipation timescale of the gas disk. Stochastic simulations show that successive collisions cannot make planetesimals grow, which may explain the formation of the debris disk observed in 99 Herculis.

On the new mechanism of planetary long-period debris formation around white dwarfs

ABSTRACT To explain the phenomenon of metal pollution of white dwarfs (WD) photospheres, we compared three main fragmentation mechanisms of small bodies (SB): tidal force, thermal destruction, and sublimation when SB fall on to WD along star-grazing orbits. The temperatures of the WDs lie in the range of 3000–15 000 K. We consider two materials, using their internal strength: crystalline ice and chondrite. We show that inside the Roche limit, ice bodies (ISB) ranging in size from 60 m to 150 km are destroyed by tidal forces. The corresponding sizes of stony bodies (SSB) range from 90 m to 130 km. Bodies of centimetre size are subject to sublimation. The thermal destruction mechanism is effective for bodies whose size lies in the interval where tidal forces and sublimation are not so effective, destroying SSBs smaller than 50 m and ISBs smaller than 1 km near stars with ${T}_\rm{eff} \le 15\, 000$ K. Such bodies are totally destroyed by thermal tensile stresses long before they reach the Roche limit. There may be observable manifestations of SB falling in the form of short-term flashes of the order of a second from SSB with sizes ≤ 100 m and WD curtaining with dust tails from ISB, causing WD dimming for a short time of the order of an hour. We conjecture that SB, moving along elongated elliptical orbits at large distances from the star, disintegrates by thermal destruction. The fragments from debris discs have nothing to do with the Roche limit.

Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA* * Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

The origin and evolution of gas in debris disks are still not well understood. Secondary gas production from cometary material or a primordial origin have been proposed. So far, observations have mostly concentrated on CO, with only a few C observations available. We overview the C and CO content of debris disk gas and test state-of-the-art models. We use new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO and C i emission, complemented by C ii data from Herschel, for a sample of 14 debris disks. This expands the number of disks with ALMA measurements of both CO and C i by 10 disks. We present new detections of C i emission toward three disks: HD 21997, HD 121191, and HD 121617. We use a simple disk model to derive gas masses and column densities. We find that current state-of-the-art models of secondary gas production overpredict the C0 content of debris disk gas. This does not rule out a secondary origin, but might indicate that the models require an additional C removal process. Alternatively, the gas might be produced in transient events rather than a steady-state collisional cascade. We also test a primordial gas origin by comparing our results to a simplified thermochemical model. This yields promising results, but more detailed work is required before a conclusion can be reached. Our work demonstrates that the combination of C and CO data is a powerful tool to advance our understanding of debris disk gas.

Small Planets around Cool Dwarfs: Enhanced Formation Efficiency of Super-Earths around M Dwarfs

Current measurements of planet population as a function of stellar mass show three seemingly contradictory signatures: close-in super-Earths are more prevalent around M dwarfs than FGK dwarfs; inner super-Earths are correlated with outer giants; and outer giants are less common around M dwarfs than FGK dwarfs. Here, we build a simple framework that combines the theory of pebble accretion with the measurements of dust masses in protoplanetary disks to reconcile all three observations. First, we show that cooler stars are more efficient at converting pebbles into planetary cores at short orbital periods. Second, when disks are massive enough to nucleate a heavy core at 5 au, more than enough dust can drift in to assemble inner planets, establishing the correlation between inner planets and outer giants. Finally, while stars of varying masses are similarly capable of converting pebbles into cores at long orbital periods, hotter stars are much more likely to harbor more massive dust disks so that the giant planet occurrence rate rises around hotter stars. Our results are valid over a wide range of parameter space for a disk accretion rate that follows . We predict a decline in mini-Neptune population (but not necessarily terrestrial planets) around stars lighter than ∼0.3–0.5M ⊙. Cold giants (≳5 au), if they exist, should remain correlated with inner planets even around lower-mass stars.

SO and SiS Emission Tracing an Embedded Planet and Compact 12CO and 13CO Counterparts in the HD 169142 Disk

Planets form in dusty, gas-rich disks around young stars, while at the same time, the planet formation process alters the physical and chemical structure of the disk itself. Embedded planets will locally heat the disk and sublimate volatile-rich ices, or in extreme cases, result in shocks that sputter heavy atoms such as Si from dust grains. This should cause chemical asymmetries detectable in molecular gas observations. Using high-angular-resolution ALMA archival data of the HD 169142 disk, we identify compact SO J = 88 − 77 and SiS J = 19 − 18 emission coincident with the position of a ∼ 2 M Jup planet seen as a localized, Keplerian NIR feature within a gas-depleted, annular dust gap at ≈38 au. The SiS emission is located along an azimuthal arc and has a morphology similar to that of a known 12CO kinematic excess. This is the first tentative detection of SiS emission in a protoplanetary disk and suggests that the planet is driving sufficiently strong shocks to produce gas-phase SiS. We also report the discovery of compact 12CO and 13CO J = 3 − 2 emission coincident with the planet location. Taken together, a planet-driven outflow provides the best explanation for the properties of the observed chemical asymmetries. We also resolve a bright, azimuthally asymmetric SO ring at ≈24 au. While most of this SO emission originates from ice sublimation, its asymmetric distribution implies azimuthal temperature variations driven by a misaligned inner disk or planet–disk interactions. Overall, the HD 169142 disk shows several distinct chemical signatures related to giant planet formation and presents a powerful template for future searches of planet-related chemical asymmetries in protoplanetary disks.