arXiv subjects

Joel Kastner

Publications and source records attributed to Joel Kastner.

At least 19 recordsLinked to original sources

arXiv2025

The Two-Zone Temperature Distribution Model: Inferences on the Structure and Composition of Dusty Protoplanetary Disks

In order to understand the mineralogy and structure of protoplanetary disks, it is important to analyze them from both an empirical spectrum-based perspective and a radiative transfer image-based perspective. In a prior paper, we set forth an empirical mineralogy mid-IR spectral model that conveyed spatial information and worked in tandem with a radiative transfer model, which formed the EaRTH Disk Model. In this article, we take the empirical portion of that model, the TZTD model, and refine it with a newly derived protoplanetary disk thermal emission formulation which uses a temperature distribution without requiring discrete integration; this simplified model uses an empirical relation between spatial distribution variables, which permits radiative transfer models to directly fit these spatial distribution variables more freely within the provided empirical constraints. We test this model against several $Spitzer~Space~Telescope$ Infrared Spectrograph (IRS) spectra, primarily transition disks, and discuss the mineralogical and structural implications of the fits, including the implications for grain growth and processing within the atmospheric zones of the disks.

astro-ph.EP↗

arXiv2025

JWST/NIRCam imaging of the Bipolar Planetary Nebula NGC 6537: the (Infra)red Spider, Revealed

We present James Webb Space Telescope (JWST) near-infrared (NIRCam) Br$α$, H$_2$, [Fe II], and PAH imaging of the molecule-rich, high-excitation bipolar planetary nebula (PN) NGC 6537 (the Red Spider), complemented by new ALMA and Chandra observations and archival HST images. The resulting multiwavelength view of the Red Spider establishes the detailed lobe/torus structure of the nebula and the mass-loss history of its progenitor star. The extinction-penetrating JWST/NIRCam Br$α$ and PAH and ALMA 3 mm continuum imaging exposes the complexity of the ionized inner nebula. JWST/NIRCam H$_2$ imaging traces the full, $\sim$1.1 pc extent of the bubble-like lobes formed by fast ($\sim$300-400 km s$^{-1}$) polar outflows, while ALMA $^{13}$CO(1-0) mapping reveals a point-symmetric, slowly ($\sim$10 km s$^{-1}$) expanding equatorial torus of radius $\sim$0.13 pc. In striking contrast, the [Fe II] image displays an extended S-shaped emission morphology that traces collisions between an active, collimated wind and slower-moving material along the lobe rims. No X-rays are detected from the nebula or its central star in deep Chandra/HRC-I imaging. However, the combined HST and JWST imaging reveals a near-IR excess at the central star indicative of emission from hot ($\sim$1000 K) circumstellar dust. We propose that interactions between the nebular progenitor star and a close companion are responsible for the ejection of NGC 6537's molecular torus, the formation of a circumbinary dust disk, and the launching of fast, wandering, collimated outflows that have inflated the polar lobe bubbles traced by near-IR H$_2$ emission and are presently generating the [Fe II]-emitting shocks.

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arXiv2025

The JWST/MIRI view of the planetary nebula NGC 6302 I.: a UV irradiated torus and a hot bubble triggering PAH formation

NGC 6302 is a spectacular bipolar planetary nebula (PN) whose spectrum exhibits fast outflows and highly ionized emission lines, indicating the presence of a very hot central star (~220,000 K). Its infrared spectrum reveals a mixed oxygen and carbon dust chemistry, displaying both silicate and polycyclic aromatic hydrocarbon (PAH) features. Using the JWST Mid-Infrared Instrument (MIRI) and Medium Resolution Spectrometer, a mosaic map was obtained over the core of NGC 6302, covering the wavelength range of 5--28 micron and spanning an area of ~18.5 arcsec x15 arcsec. The spatially resolved spectrum reveals ~200 molecular and ionized lines from species requiring ionisation potentials of up to 205 eV. The spatial distributions highlight a complex structure at the nebula's centre. Highly ionized species such as Mg VII and Si VII show compact structures, while lower-ionization species such as H^+ extend much farther outwards, forming filament-defined rims that delineate a bubble. Within the bubble, the H^+ and H_2 emission coincide, while the PAH emission appears farther out, indicating an ionization structure distinct from typical photodissociation regions, such as the Orion Bar. This may be the first identification of a PAH formation site in a PN. This PN appears to be shaped not by a steady, continuous outflow, but by a series of dynamic, impulsive bubble ejections, creating local conditions conducive to PAH formation. A dusty torus surrounds the core, primarily composed of large (micron-sized) silicate grains with crystalline components. The long-lived torus contains a substantial mass of material, which could support an equilibrium chemistry and a slow dust-formation process.

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arXiv2024

Spectroscopic Observations of the Galex Nearby Young Star Survey Sample. 1. Nearby Moving Group Candidates

The GALEX Nearby Young Star Search (GALNYSS) yielded the identification of more than 2000 late-type stars that, based on their ultraviolet and infrared colors and pre-Gaia proper motions, are potentially of age < 200 Myr and lie within ~120 pc of Earth. We present the results of a campaign of medium- and high-resolution optical spectroscopy of 471 GALNYSS stars aimed at confirming their youth and their potential membership in nearby young stellar moving groups. We present radial velocity (RV), Li absorption, and H-$α$ emission measurements for these spectroscopically observed GALNYSS stars, and assess their Li absorption and optical emission-line properties and infrared excesses. Our RV measurements are combined with literature and Gaia DR3 RV measurements and Gaia DR3 astrometry and photometry to obtain the spatiokinematics and color-magnitude positions of GALNYSS stars. We use these results to assess membership in the TW Hya, Tuc-Hor, Carina, Columba, and Argus Associations and the $β$-PMG and AB Dor moving groups. We have identified 132 stars as candidate members of these seven groups; roughly half of these candidates are newly identified on the basis of data presented here. At least one-third of the 132 candidates are spectroscopic and/or photometric binaries and/or have comoving (visual) binary companions in Gaia DR3. The contingent of young, low-mass stars in the solar vicinity we identify here should provide excellent subjects for future direct imaging exoplanet surveys and studies of the early evolution of low-mass stars and their planetary progeny.

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arXiv2024

High-Speed Outflows and Dusty Disks during the AGB to PN Transition: The PANORAMA survey

As mass-losing asymptotic giant branch (AGB) stars evolve to planetary nebulae (PNe), the mass outflow geometries transform from nearly spherical to extreme aspherical. The physical mechanisms governing this transformation are widely believed to be linked to binarity and the associated production of disks and fast jets during transitional (post-AGB) evolutionary stages. We are carrying out a systematic ALMA survey ($P$re-planet$A$ry $N$ebulae high-angular-res$O$lution su$R$vey with $A$L$MA$ or PANORAMA) of a representative sample of bipolar and multipolar post-AGB objects. We have obtained high angular-resolution (0".1-0".4) observations of the CO(3--2) and/or 6--5 emission in order to probe the spatio-kinematic structure of the collimated outflows and the central disk/torii. The results are remarkable, generally showing the presence of bipolar or multipolar high-velocity outflows, dense toroidal waists, and in one case, a geometrically-thin circular ring around the central bipolar nebula. A high degree of point-symmetry characterizes the morphology of the mass ejecta. In this contribution, we present these and other highlights from our survey. We aim to use 2D/3D radiative transfer modeling in order to derive accurate outflow momenta, masses and mass-loss rates for our sample, and build hydrodynamical models that can explain the observed spatio-kinematic structures. These results will then be used to distinguish between different classes of PN-shaping binary interaction models.

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arXiv2024

A Survey of Protoplanetary Disks Using the Keck/NIRC2 Vortex Coronagraph

Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations of protoplanetary disks in the millimeter continuum have shown a variety of radial gaps, cavities, and spiral features. These substructures may be signposts for ongoing planet formation, and therefore these systems are promising targets for direct imaging planet searches in the near-infrared. To this end, we present results from a deep imaging survey in the $L'$-band (3.8 $μ$m) with the Keck/NIRC2 vortex coronagraph to search for young planets in 43 disks with resolved features in the millimeter continuum or evidence for gaps/central cavities from their spectral energy distributions. Although we do not detect any new point sources, using the vortex coronagraph allows for high sensitivity to faint sources at small angular separations (down to ${\sim}$0$^{\prime\prime}$.1), allowing us to place strong upper limits on the masses of potential gas giant planets. We compare our mass sensitivities to the masses of planets derived using ALMA observations, and while we are sensitive to $\sim$1 M$_{Jup}$ planets in the gaps in some of our systems, we are generally not sensitive to planets of the masses expected from the ALMA observations. In addition to placing upper limits on the masses of gas giant planets that could be interacting with the dust in the disks to form the observed millimeter substructures, we are also able to map the micron-sized dust as seen in scattered light for 8 of these systems. Our large sample of systems also allows us to investigate limits on planetary accretion rates and disk viscosities.

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arXiv2024

The Empirical and Radiative Transfer Hybrid (EaRTH) Disk Model: Merging Analyses of Protoplanetary Dust Disk Mineralogy and Structure

Our understanding of how exoplanets form and evolve relies on analyses of both the mineralogy of protoplanetary disks and their detailed structures; however, these key complementary aspects of disks are usually studied separately. We present initial results from a hybrid model that combines the empirical characterization of the mineralogy of a disk, as determined from its mid-infrared spectral features, with the MCFOST radiative transfer disk model, a combination we call the EaRTH Disk Model. With the results of the mineralogy detection serving as input to the radiative transfer model, we generate mid-infrared spectral energy distributions (SEDs) that reflect both the mineralogical and structural parameters of the corresponding disk. Initial fits of the SED output by the resulting integrated model to Spitzer Space T elescope mid-infrared (IRS) spectra of the protoplanetary disk orbiting the nearby T Tauri star MP Mus demonstrate the potential advantages of this approach by revealing details like the dominance of micron-sized olivine and micron-sized forsterite in this dusty disk. The simultaneous insight into disk composition and structure provided by the EaRTH Disk methodology should be directly applicable to the interpretation of mid-infrared spectra of protoplanetary disks that will be produced by the James Webb Space Telescope.

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arXiv2024

The Nature of X-Rays from Young Stellar Objects in the Orion Nebula Cluster -- A Chandra HETGS Legacy Project

The Orion Nebula Cluster (ONC) is the closest site of very young ($\sim$ 1 Myrs) massive star formation. The ONC hosts more than 1600 young and X-ray bright stars with masses ranging from $\sim$ 0.1 to 35 $M_\odot$. The Chandra HETGS Orion Legacy Project observed the ONC with the Chandra high energy transmission grating spectrometer (HETGS) for $2.1\,$Ms. We describe the spectral extraction and cleaning processes necessary to separate overlapping spectra. We obtained 36 high resolution spectra which includes a high brilliance X-ray spectrum of $θ^1$ Ori C with over 100 highly significant X-ray lines. The lines show Doppler broadening between 300 and $400\;\mathrm{km}\;\mathrm{s}^{-1}$. Higher spectral diffraction orders allow us to resolve line components of high Z He-like triplets in $θ^1$ Ori C with unprecedented spectral resolution. Long term light curves spanning $\sim$20 years show all stars to be highly variable, including the massive stars. Spectral fitting with thermal coronal emission line models reveals that most sources show column densities of up to a few times $10^{22}\,$cm$^{-2}$ and high coronal temperatures of 10 to 90 MK. We observe a bifurcation of the high temperature component where some stars show a high component of 40 MK, while others show above 60 MK indicating heavy flaring activity. Some lines are resolved with Doppler broadening above our threshold of $\sim200\;\mathrm{km}\;\mathrm{s}^{-1}$, up to $500\;\mathrm{km}\;\mathrm{s}^{-1}$. This data set represents the largest collection of HETGS high resolution X-ray spectra from young pre-MS stars in a single star-forming region to date.

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arXiv2022

R2-D2: Roman and Rubin -- From Data to Discovery

The NASA Nancy Grace Roman Space Telescope (Roman) and the Vera C. Rubin Observatory Legacy Survey of Space and Time (Rubin), will transform our view of the wide-field sky, with similar sensitivities, but complementary in wavelength, spatial resolution, and time domain coverage. Here we present findings from the AURA Roman+Rubin Synergy Working group, charged by the STScI and NOIRLab Directors to identify frontier science questions in General Astrophysics, beyond the well-covered areas of Dark Energy and Cosmology, that can be uniquely addressed with Roman and Rubin synergies in observing strategy, data products and archiving, joint analysis, and community engagement. This analysis was conducted with input from the community in the form of brief (1-2 paragraph) "science pitches" (see Appendix), and testimony from "outside experts" (included as co-authors). We identify a rich and broad landscape of potential discoveries catalyzed by the combination of exceptional quality and quantity of Roman and Rubin data, and summarize implementation requirements that would facilitate this bounty of additional science with coordination of survey fields, joint coverage of the Galactic plane, bulge, and ecliptic, expansion of General Investigator and Target of Opportunity observing modes, co-location of Roman and Rubin data, and timely distribution of data, transient alerts, catalogs, value-added joint analysis products, and simulations to the broad astronomical community.

astro-ph.IM↗

arXiv2020

Evidence for magnetic activity at starbirth: a powerful X-ray flare from the Class 0 protostar HOPS 383

Context. Class 0 protostars represent the earliest evolutionary stage of solar-type stars, during which the majority of the system mass resides in an infalling envelope of gas and dust and is not yet in the central, nascent star. Although X-rays are a key signature of magnetic activity in more evolved protostars and young stars, whether such magnetic activity is present at the Class 0 stage is still debated. Aims. We aim to detect a bona fide Class 0 protostar in X-rays. Methods. We observed HOPS 383 in 2017 December in X-rays with the Chandra X-ray Observatory ($\sim$84 ks) and in near-infrared imaging with the Southern Astrophysical Research telescope. Results. HOPS 383 was detected in X-rays during a powerful flare. This hard (E > 2 keV) X-ray counterpart was spatially coincident with the northwest 4 cm component of HOPS 383, which would be the base of the radio thermal jet launched by HOPS 383. The flare duration was $\sim$3.3 h; at the peak, the X-ray luminosity reached $\sim$4 x 1E31 erg s --1 in the 2-8 keV energy band, a level at least an order of magnitude larger than that of the undetected quiescent emission from HOPS 383. The X-ray flare spectrum is highly absorbed (NH $\sim$ 7 x 1E23 cm --2), and it displays a 6.4 keV emission line with an equivalent width of $\sim$1.1 keV, arising from neutral or low-ionization iron. Conclusions. The detection of a powerful X-ray flare from HOPS 383 constitutes direct proof that magnetic activity can be present at the earliest formative stages of solar-type stars.

astro-ph.HE↗

arXiv2019

Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation

The isotopic ratio of nitrogen measured in primitive Solar System bodies shows a broad range of values, the origin of which remains unknown. One key question is whether these isotopic reservoirs of nitrogen predate the comet formation stage or are posterior to it. Another central question is elucidating the processes that can produce the observed variations in the 14N/15N isotopic ratio. Disks that orbit pre-main-sequence stars provide unique opportunities for observing the chemical content of analogs of the protosolar nebula and therefore for building a comprehensive scenario for the origin of nitrogen in the Solar System and in planet-forming disks. We present spectrally and spatially resolved observations of the hyperfine structure of the 4-3 rotational transition of HCN and its main isotopologs H13CN and HC15N in the disk orbiting the 8 Myr old TTauri star TW Hya. The spatially averaged HCN/H13CN and HCN/HC15N abundance ratios are 86+/-4 and 223+/-21, respectively. The latter value is significantly lower than the CN:C15N ratio 323+/-30 in this disk and thus provides the first evidence that two isotopic reservoirs of nitrogen are present in a disk at the stage of giant planet and comet formation. Furthermore, we find clear evidence for an increase in the ratio of HCN to HC15N with radius. The ratio in the outer disk is 339+/-28, in excellent agreement with direct measurements in the local interstellar medium, and with the bulk nitrogen isotopic ratio predicted from galactic evolution calculations. In the comet formation region at r=20 au, the ratio is a factor ~3 lower, 121+/-11. This radial increase qualitatively agrees with the scenario in which selective photodissociation of N 2 is the dominant fractionation process. However, our isotopic ratios and kinetic temperature of the HCN-emitting layers quantitatively disagree with models of nitrogen chemistry in disks.

astro-ph.GA↗

arXiv2019

LRP2020: The Opportunity of Young Nearby Associations with the Advent of the Gaia Mission

This white paper proposes leveraging high-quality Gaia data available to the worldwide scientific community and complement it with support from Canadian-related facilities to place Canada as a leader in the fields of stellar associations and exoplanet science, and to train Canadian highly qualified personnel through graduate and post-graduate research grants. Gaia has sparked a new era in the study of stellar kinematics by measuring precise distances and proper motions for 1.3 billion stars. These data have already generated more than 1700 scientific papers and are guaranteed to remain the source of many more papers for the upcoming decades. More than 900 new age-calibrated young low-mass stars have already been discovered as a direct consequence of the second Gaia data release. Some of these may already be host stars to known exoplanet systems or may become so with the progress of the TESS mission that is expected to discover 10,000 nearby transiting exoplanets in the upcoming decade. This places Canada in a strategic position to leverage Gaia data because it has access to several high-resolution spectrometers on 1-4 m class telescopes (e.g. The ESPaDOnS, SPIRou and NIRPS), that would allow to quickly characterize this large number of low-mass stars and their exoplanet systems. This white paper describes the opportunity in such scientific projects that could place Canada as a leader in the fields of stellar associations and exoplanets.

astro-ph.SR↗

arXiv2019

The Nearby, Young, Chi1 Fornacis Cluster: Membership, Age, and an Extraordinary Ensemble of Dusty Debris Disks

Only four star clusters are known within ~100 pc of Earth. Of these, the Chi1 For cluster has barely been studied. We use the Gaia DR2 catalog and other published data to establish the cluster membership, structure, and age. The age of and distance to the cluster are ~40 Myr and 104 pc, respectively. A remarkable, unprecedented, aspect of the cluster is the large percentage of M-type stars with warm excess infrared emission due to orbiting dust grains -- these stars lie in an annulus that straddles the tidal radius of the cluster. The Chi1 For cluster appears to be closely related to two extensive, previously known, groups of co-moving, coeval stars (the Tucana-Horologium and Columba Associations) that are spread over much of the southern sky. While Tuc-Hor and Chi1 For are comoving and coeval, the difference in the frequency of their warm dusty debris disks at M-type stars could hardly be more dramatic.

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arXiv2019

X-ray Studies of Exoplanets: A 2020 Decadal Survey White Paper

Over the last two decades, the discovery of exoplanets has fundamentally changed our perception of the universe and humanity's place within it. Recent work indicates that a solar system's X-ray and high energy particle environment is of fundamental importance to the formation and development of the atmospheres of close-in planets such as hot Jupiters, and Earth-like planets around M stars. X-ray imaging and spectroscopy provide powerful and unique windows into the high energy flux that an exoplanet experiences, and X-ray photons also serve as proxies for potentially transfigurative coronal mass ejections. Finally, if the host star is a bright enough X-ray source, transit measurements akin to those in the optical and infrared are possible and allow for direct characterization of the upper atmospheres of exoplanets. In this brief white paper, we discuss contributions to the study of exoplanets and their environs which can be made by X-ray data of increasingly high quality that are achievable in the next 10--15 years.

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arXiv2019

High-Energy Photon and Particle Effects onExoplanet Atmospheres and Habitability

It is now recognized that energetic stellar photon and particle radiation evaporates and erodes planetary atmospheres and controls upper atmospheric chemistry. Key exoplanet host stars will be too faint at X-ray wavelengths for accurate characterization using existing generation and future slated X-ray telescopes. Observation of stellar coronal mass ejections and winds are also beyond current instrumentation. In line with theCommittee on an Exoplanet Science Strategy recognition that holistic observational approaches are needed, we point out here that a full understanding of exoplanet atmospheres, their evolution and determination of habitability requires a powerful high-resolution X-ray imaging and spectroscopic observatory. This is the only capability that can: (1) characterize by proxy the crucial, difficult to observe, EUV stellar flux, its history and its variations for planet hosting stars; (2) observe the stellar wind; (3) detect the subtle Doppler signatures of coronal mass ejections.

astro-ph.HE↗

arXiv2018

The Dissolution of Clusters: What Can We Learn from Nearby, UV-bright, Overluminous Field Stars?

The past two decades have seen dramatic progress in our knowledge of the population of young stars of age <200 Myr that lie within 150 pc of the Sun. These nearby, young stars, most of which are found in loose, comoving groups, provide the opportunity to explore (among many other things) the dissolution of stellar clusters and their diffusion into the field star population. In the age of Gaia, this potential can now be fully exploited. We have identified, and are now investigating, a sample of nearly 400 Galex UV-selected late-type (K and early-M) field stars with Gaia-based distances <120 pc and isochronal ages <=80 Myr (even if binaries). Only a small percentage (<10%) of stars among this (kinematically unbiased) sample can be confidently associated with established nearby, young moving groups (NYMGs). The majority display anomalous kinematics, relative to the known NYMGs. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population. We discuss the implications and caveats of such a hypothesis---including the intriguing fact that, in addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within 50 pc appear surprisingly X-ray faint.

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arXiv2017

The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781

We perform a comprehensive analysis of the planetary nebula (PN) NGC6781 to investigate the physical conditions of each of its ionized, atomic, and molecular gas and dust components and the object's evolution, based on panchromatic observational data ranging from UV to radio. Empirical nebular elemental abundances, compared with theoretical predictions via nucleosynthesis models of asymptotic giant branch (AGB) stars, indicate that the progenitor is a solar-metallicity, 2.25-3.0 Msun initial-mass star. We derive the best-fit distance of 0.46 kpc by fitting the stellar luminosity (as a function of the distance and effective temperature of the central star) with the adopted post-AGB evolutionary tracks. Our excitation energy diagram analysis indicate high excitation temperatures in the photodissociation region (PDR) beyond the ionized part of the nebula, suggesting extra heating by shock interactions between the slow AGB wind and the fast PN wind. Through iterative fitting using the Cloudy code with empirically-derived constraints, we find the best-fit dusty photoionization model of the object that would inclusively reproduce all of the adopted panchromatic observational data. The estimated total gas mass (0.41 Msun) corresponds to the mass ejected during the last AGB thermal pulse event predicted for a 2.5 Msun initial-mass star. A significant fraction of the total mass (about 70 percent) is found to exist in the PDR, demonstrating the critical importance of the PDR in PNe that are generally recognized as the hallmark of ionized/H+ regions.

astro-ph.SR↗

arXiv2017

Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue

Isotopic ratios are keys to understanding the origin and early evolution of the solar system in the context of Galactic nucleosynthesis. The large range of measured $^{14}$N/$^{15}$N isotopic ratios in the solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. We have directly measured a C$^{14}$N/C$^{15}$N abundance ratio of 323$\pm$30 in the disk orbiting the nearby young star TW Hya. This value, which is in good agreement with nitrogen isotopic ratios measured for prestellar cores, likely reflects the primary present-day reservoir of nitrogen in the solar neighbourhood. These results support models invoking novae as primary $^{15}$N sources as well as outward migration of the Sun over its lifetime, and suggest that comets sampled a secondary, $^{15}$N-rich reservoir during solar system formation.

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