arXiv subjects

Adolfo S. Carvalho

Publications and source records attributed to Adolfo S. Carvalho.

At least 19 recordsLinked to original sources

arXiv2026

Conspicuous Gas, Cryptic Dust: Spectroscopy and Chromaticity of Complex Periodic Variables

Complex periodic variables (CPVs) are young low-mass stars whose light curves show periodic dips indicative of transiting corotating material. The origin and composition of this material are unclear. Here we present new optical and near-infrared spectroscopy and photometry of four CPVs from Magellan, Keck, Hale, MuSCAT1, MuSCAT2, Tierras, KeplerCam, and TESS. The spectra imply that CPVs host magnetically bound circumstellar plasma clumps, on the basis of sinusoidal-in-time Balmer emission out of transit, and Balmer dimming during transit. Yet large night-to-night changes in circumstellar hydrogen emissivity occur without clear changes in light curve morphology, suggesting that the sharp flux dips are caused not by circumstellar plasma but by dust. Optical chromaticities (depth proportional to $λ^{-β}$, with $β$ = 0.79 $\pm$ 0.16) support this, but the power law breaks in the near-infrared, where a single power law under-predicts the depths observed at 2.1 microns. We therefore favor dips caused by dusty plasma clumps with opaque cores and optically thin halos, though we cannot rule out models in which the dust properties vary per-star or per-epoch. Observations at wavelengths greater than 2 microns and less than 0.4 microns would test this interpretation and clarify the dust's origin.

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arXiv2026

VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales

We present a multiwavelength analysis of the radio transient VT J1906+0849, discovered as a 70 mJy source in Epoch 1 of the Very Large Array Sky Survey (VLASS), 21 yr after an NRAO VLA Sky Survey (NVSS) non-detection. Radio observations reveal the source was first detected in 2005, peaking at $\gtrsim200$ mJy in 2014, then declining until a late 2025 rebrightening. The transient sits at a Galactic latitude of $\approx0.74^\circ$ and the properties of its optical-infrared counterpart support a Galactic origin. At $d\gtrsim15$ kpc, the extreme radio luminosity is likely powered by sustained accretion onto a compact object. However, a Swift-XRT non-detection shows it is X-ray faint relative to the Galactic X-ray binary population, and the radio emission is distinct from X-ray binaries in its temporal and spectral behavior. Broadband radio spectra suggest synchrotron self-absorption, but size constraints from equipartition and very long baseline interferometry show little to no expansion in the radio-emitting region over 5+ yr, despite significant spectral evolution. Near-infrared spectroscopy reveals a single broad emission line with a stable centroid but variable width and luminosity. We attribute this feature to blueshifted Br$γ$ tracing a persistent asymmetric $\approx2000$ km s$^{-1}$ outflow. These properties are unlike any previously identified Galactic radio source. One possible interpretation is that VT J1906+0849 is a young analog of the microquasar SS 433, with a dense disk wind confining a continuously powered synchrotron outflow. This jet-wind interaction explains the compact, slowly expanding radio source and may contribute to the absence of bright X-ray emission.

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arXiv2026

V7995 Sgr: A New FU Orionis Accretion Outburst Near NGC 6589/6590

We announce a new FU Orionis type outburst that reached peak brightness in late 2024, following a steep 4.6 month photometric rise of -2.85 mag in the $r$ band. This rapid brightening at all wavelengths was preceeded in the infrared by a much shallower rise over 4 years. The progenitor object was an unstudied young stellar object having a flat-spectrum type spectral energy distribution, and extended nebulosity. We present multi-wavelength lightcurves covering the photometric low-state, the outburst, and early post-outburst epochs. Optical imaging shows a concurrent brightening of the extended nebular environment. We also present follow-up optical/near-infrared spectroscopy taken 1.5 years after the inferred photometric peak. The spectra confirm an FU Ori type outburst. The outburst source exhibits a mixed-temperature absorption spectrum, formed in an accretion disk, and it shows several line species with blueshifted absorption profiles that are formed in a strong wind.

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arXiv2026

A Parameterized YSO Accretion Disk Model with Increasing Accretion Rate: Predicted Outburst Lightcurves

A sub-class among Young Stellar Objects (YSOs), known as FU Ori type stars, undergo sudden rises in luminosity by several orders of magnitude on timescales of a few months to a few years, and decay back to quiescence on timescales of a few decades. Modelling the light curves of these objects is crucial to understanding how different components of these accretion disk systems evolve during outburst. For this purpose, we use a parametric model that couples the stellar photospheric emission, magnetospheric accretion shocks, an irradiated dust disk, and a viscously heated gas disk. We adopt time-dependent accretion rate profiles that mimic the observed morphologies of FU Ori outburst light curves, and we use the accretion model infrastructure to simulate multi-band light curves, as well as color curves. The model enables us to study how different components dominate the flux in each band over the course of an outburst, providing insight into star-magnetosphere-disk interactions throughout the outburst cycle. We find that throughout an accretion outburst, red optical and near-infrared lightcurves generally follow the same or very similar form as the input accretion profile, being sensitive to heating in the accretion shocks and inner gas disk, while mid-infrared lightcurves are more responsive to the location and heating of the innermost dust disk.

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arXiv2026

Quantitative Spectroscopic Diagnostics for FU Orionis-Type Young Stellar Objects

We present near-infrared spectroscopic diagnostics that can be used to identify FU Orionis stars (FUOrs). FUOrs are young stellar objects (YSOs) that are currently in a state of extreme outburst, caused by enhanced mass {inflow} from their accretion disks. The disks give FUOrs a distinct multi-temperature optical and infrared spectrum. Considering both the predicted spectrum from a disk atmosphere model, and existing spectral diagnostics from the literature, we identify key atomic and molecular features for characterizing FUOrs. Some of the chosen features are proxies for temperature, others are sensitive to surface gravity, and still others probe disk winds. Using the Palomar Observatory/Hale Telescope TripleSpec spectrograph, we gathered near-infrared spectra of 28 known FUOrs. We use standard equivalent widths to determine the strength of atomic lines and we design several band ratios for measuring molecular features. We compare the measurements between our spectra and a control sample of late-type dwarfs and evolved stars from the Infrared Telescope Facility Spectral Library. By considering the relative distributions of these samples in our defined spectral diagnostics, we propose a number of parameter spaces that can distinguish FUOr disks from normal stars. The rate of discovery of FUOr candidates has increased significantly in recent years, largely due to the increasing prevalence of time-domain surveys. Our proposed diagnostics will allow new photometric candidates to be confirmed or refuted as such.

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arXiv2026

The FUor Mass Distribution Matches the Solar Neighborhood IMF: Evidence for a Universal Eruptive Phase

Eruptive accretion events are expected to play an important role in the mass buildup stage of individual star formation. FU Ori objects (FUors) experience the most extreme eruptive outbursts, which raise the accretion rate of the disk from $10^{-9}-10^{-8} \ M_\odot \ \mathrm{yr}^{-1}$ to $10^{-5}-10^{-4} \ M_\odot \ \mathrm{yr}^{-1}$ and last for decades. During an outburst, the disk is approximately 100 times brighter than the star, making direct study of the central star impossible. However, the disk is expected to be in Keplerian rotation around the star, enabling indirect constraints on properties of the central source via observations of the disk. Using $1-2.4 \ μ$m high resolution spectra of several tens of FUors, we demonstrate the expected Keplerian rotation in their inner disks. We then adopt a Keplerian rotational broadening profile to model the line profiles of spectral lines, and focussing on the H-band region, we infer the mass distribution of FUors. We finally show that this mass distribution is consistent with inferred Solar neighborhood initial mass functions, suggesting all young stars undergo a period of FUor outbursts in their pre main-sequence evolution.

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arXiv2025

A New FU Orionis Accretion Outburst in the W5 HII Region

We announce a recently detected outburst that is currently only a few months old, and probably of FU Orionis type. The progenitor to the outburst was an emission-line, flat-spectrum SED young stellar object located in the W5 region, though somewhat outside the main star formation action. We present optical, near-infrared, and mid-infrared lightcurves that illustrate the quiescent state of [KAG2008] 13656 and its subsequent$Δr \approx -4$ mag and $ΔJ\approx -3$ mag outburst over $\sim$75 days in late-2025. Follow-up optical and near-infrared spectroscopy confirms the expected features from an FU Ori disk and outflow.

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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.

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arXiv2025

The Near-Ultraviolet Spectra of FU Orionis Accretion Disks

We present the results of the first high-sensitivity NUV (1800 to 3200 Å) survey of FU Ori objects, using the \textit{Hubble Space Telescope} (HST) STIS spectrograph. We compare new low resolution spectra for 6 sources with predictions from accretion disk models and find that all show emission in excess of the disk model spectrum. The physical properties of the NUV emission excess are very consistent among the sample, with a mean luminosity of $10^{-1.11 \pm 0.4} \ L_\odot$ and temperature of $16400 \pm 2600$ K -- despite spanning 0.9 dex in $M_*$, 1.3 dex in $\dot{M}$, and 0.7 dex in $L_\mathrm{acc}$. We use the spectra to conclusively rule out the existence of a hot boundary layer in FU Ori accretion disks. We then discuss the source of the excess emission in the context of recent simulations of FU Ori outbursts and boundary layer accretion. The UV spectra also show the often-seen \ion{C}{2}] 2326 Å multiplet and \ion{Mg}{2} 2796/2803 Å doublet, as well as the unusual \ion{Fe}{2}] 2507/2509 Å doublet, a feature that is not seen in the existing UV spectra of other young stellar objects. We measure and compare the luminosities of these lines in outbursting with those in non-outbursting objects.

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arXiv2025

The Hot Inner AU of V883 Ori

The V883 Ori system is a rapidly accreting young stellar object that has been used as a laboratory for studying the molecular inventory of young circumstellar disks with high luminosity. We simultaneously fit high resolution spectroscopy and medium resolution spectrophotometry of the system to constrain the physical conditions in the inner au. Using our thin viscous accretion disk model, we find $\dot{M} = 10^{-3.9 \pm 0.2} \ M_\odot$ yr$^{-1}$, $R_\mathrm{inner} = 5.86 \pm 1 \ R_\odot$, $i = 38.2 \pm 3$ degrees and $A_V = 20.8 \pm 0.7$ mag, resulting in an accretion luminosity of 458 $L_\odot$ and maximum disk temperature of 7045 K. The optical portion of the SED greatly exceeds the flux level expected for a highly extincted accretion disk. We propose that the excess emission arises from a contribution due to scattering of the accretion disk spectrum off nearby envelope material that is viewed along a less-extincted line of sight. Additionally, we use photometric observations spanning 137 years to demonstrate that the source has accreted at least 18 $M_\mathrm{Jup}$ of disk material to date. Finally, we discuss the importance of considering both the viscous heating from the midplane and the consequent irradiation effects on the outer disk when modeling the temperature structure to reproduce millimeter-wavelength observations.

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arXiv2025

FUOr-Aur 0544+3330: A New YSO Outburst in the Outskirts of Auriga OB1, Viewed Face-On

We present a newly appreciated FU Ori outburst event that began in 2019 and reached a peak in early 2021. Suspected young stellar object WISE J054452.25+333009.6 experienced substantial brightening, in excess of $-5$ mag at optical wavelengths and $-2.5$ mag at mid-infrared wavelengths. The time from near-quiescence to peak brightness was approximately 24 months. Optical and near-infrared spectra confirm that the outbursting source (hereby designated FUOr-Aur 0544+3330) shows all the hallmarks of the FU Ori class, including the Li I indicator of stellar youth. The mix of ionized and neutral atomic lines, alongside prominent molecular absorption features, is consistent with the expected change in spectral type from earlier in the optical to later-type in the near-infrared. The closest analog among well-studied FU Ori objects is V1515 Cyg. Both sources have unusually narrow-lined absorption spectra that can be explained by a face-on disk orientation, such that disk-broadening is minimized and wind-induced blueshift (in e.g. H$α$, NaD, Ca II) is maximized. Both the optical through infrared spectral energy distribution and high-resolution spectrum are well-fit by a pure-accretion disk model. Adopting a distance of $d=1.5$ kpc, the accretion and central star parameters are: $\dot{M} = 10^{-5.48}$ $M_\odot$ yr$^{-1}$, $M_* = 0.17 \ M_\odot$, and $R_\mathrm{inner} = 1.04 \ R_\odot$. Other fitted values are disk inclination $i=5.9$ deg and source extinction $A_V=1.83$ mag. These parameters yield accretion luminosity $L_\mathrm{acc} = 8.4\ L_\odot$ and maximum disk temperature $T_{\rm{max}} = 6218$ K.

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arXiv2025

Rosette Nebula Outburst Gaia 24djk from the Young Stellar Object V557 Mon

A previously faint young stellar object (YSO), V557 Mon, rapidly brightened in late 2024 and is currently at least $ΔG=3.3$ magnitudes brighter than its typical pre-outburst brightness. The ongoing outburst is identified in the Gaia Alerts system as Gaia24djk. We obtained a 1-2.5 $μ$m spectrum of the object and find the spectrum is dominated by line emission and continuum excess consistent with rapid YSO accretion, similar to the star EX Lup during its outburst state. We speculate that the burst, which has not yet reached its peak brightness, may become an FU Ori outburst, which would be evidenced by the emission spectrum turning into an absorption spectrum.

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arXiv2024

Bolometric Corrections for FU Ori Object Accretion Disk Luminosities

The accretion luminosity of an FU Ori disk is a fundamental system parameter, but a challenging one to estimate for all but the most well-studied systems. FU Ori objects are dynamically evolving accretion disks, especially close in time to the outburst epoch. They have a complex multi-temperature disk structure that results in distinctly shaped, broad SEDs. Detailed spectroscopic analysis is required for simultaneous constraint on relevant physical parameters such as the central stellar mass, inner disk radius, disk inclination, and disk accretion rate. However, outbursting systems that are deeply embedded and/or distant may be limited to only photometric measurement, and over only a narrow range of wavelengths. The bolometric corrections necessary to estimate accretion luminosities are not straightforward, and in particular can not be adopted from existing literature on isotropically radiating stars. We present bolometric corrections specific to astrophysical accretion disks for a variety of filters in ongoing and upcoming all-sky surveys.

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arXiv2024

Robust support for semi-automated reductions of Keck/NIRSPEC data using PypeIt

We present a data reduction pipeline (DRP) for Keck/NIRSPEC built as an addition to the PypeIt Python package. The DRP is capable of reducing multi-order echelle data taken both before and after the detector upgrade in 2018. As part of developing the pipeline, we implemented major improvements to the capabilities of the PypeIt package, including manual wavelength calibration for multi-order data and new output product that returns a coadded spectrum order-by-order. We also provide a procedure for correcting telluric absorption in NIRSPEC data by using the spectra of telluric standard stars taken near the time of the science spectra. At high resolutions, this is often more accurate than modeling-based approaches.

astro-ph.IM↗

arXiv2024

The FUV Spectrum of FU Ori South

The eruptive YSO FU Ori is the eponym of its variable class. FU Ori stars are known to undergo outbursts with amplitudes of $>4$ magnitudes in the $V$ band and durations of several decades. Interaction with a binary companion is one proposed outburst trigger, so understanding both components of the FU Ori system is crucial. A recent HST/STIS observation of the FU Ori system clearly resolves its North and South components. We report here on the spectrum of FU Ori South. We detect NUV continuum emission but no FUV continuum, although several bright emission lines consistent with those seen in T Tauri stars are present. The presence of the C II] 2325 multiplet and many H$_2$ lines indicate active accretion. We estimate the extinction to the source and find that the UV spectrum favors $A_V < 4$, contrary to past estimates based on the NIR spectrum.

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arXiv2024

An FUV-detected Accretion Shock at the Star-Disk Boundary of FU Ori

FU Ori objects are the most extreme eruptive young stars known. Their 4 to 5 magnitude photometric outbursts last for decades and are attributed to a factor of up to 10,000 increase in the stellar accretion rate. The nature of the accretion disk-to-star interface in FU Ori objects has remained a mystery for decades. To date, attempts to directly observe a shock or boundary layer have been thwarted by the apparent lack of emission in excess of the accretion disk photosphere down to $λ= 2300$ Å. We present a new NUV and the first high-sensitivity FUV spectrum of FU Ori. The FUV continuum is detected for the first time and, at $λ= 1400$ Å, is more than $10^4$ times brighter than predicted by a viscous accretion disk. We interpret the excess as arising from a shock at the boundary between the disk and the stellar surface. We model the shock emission as a blackbody and find that the temperature of the shocked material is $T_\mathrm{FUV} \approx 16,000 \pm 2000$ K. The shock temperature corresponds to an accretion flow along the surface of the disk that reaches a velocity of 40 km s$^{-1}$ at the boundary, consistent with predictions from simulations.

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arXiv2024

A Dust-Trapping Ring in the Planet-Hosting Disk of Elias 2-24

Rings and gaps are among the most widely observed forms of substructure in protoplanetary disks. A gap-ring pair may be formed when a planet carves a gap in the disk, which produces a local pressure maximum following the gap that traps inwardly drifting dust grains and appears as a bright ring due to the enhanced dust density. A dust-trapping ring would provide a promising environment for solid growth and possibly planetesimal production via the streaming instability. We present evidence of dust trapping in the bright ring of the planet-hosting disk Elias 2-24, from the analysis of 1.3 mm and 3 mm ALMA observations at high spatial resolution (0.029 arcsec, 4.0 au). We leverage the high spatial resolution to demonstrate that larger grains are more efficiently trapped and place constraints on the local turbulence ($8 \times 10^{-4} < α_\mathrm{turb} < 0.03$) and the gas-to-dust ratio ($Σ_g / Σ_d < 30$) in the ring. Using a scattering-included marginal probability analysis we measure a total dust disk mass of $M_\mathrm{dust} = 13.8^{+0.7}_{-0.5} \times 10^{-4} \ M_\odot$. We also show that at the orbital radius of the proposed perturber, the gap is cleared of material down to a flux contrast of 10$^{-3}$ of the peak flux in the disk.

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arXiv2024

An Expanding Accretion Disk and a Warm Disk Wind As Seen In the Spectral Evolution of HBC 722

We present a comprehensive analysis of the post-outburst evolution of the FU Ori object HBC 722 in optical/near-infrared (NIR) photometry and spectroscopy. Using a modified viscous accretion disk model, we fit the outburst epoch SED to determine the physical parameters of the disk, including $\dot{M}_\mathrm{acc} = 10^{-4.0} \ M_\odot$ yr$^{-1}$, $R_\mathrm{inner} = 3.65 \ R_\odot$, $i = 79^\circ$, and a maximum disk temperature of $T_\mathrm{max} = 5700$ K. We then use a decade of optical/NIR spectra to demonstrate a changing accretion rate drives the visible-range photometric variation, while the NIR shows the outer radius of the active accretion disk expands outward as the outburst progresses. We also identify the major components of the disk system: a plane-parallel disk atmosphere in Keplerian rotation and a 2-part warm disk wind that is collimated near the star and wide-angle at larger radii. The wind is traced by classic wind lines, and appears as a narrow, low-velocity, deep absorption component in several atomic lines spanning the visible spectrum and in the CO 2.29$μ$m band. We compare the wind lines to those computed from wind models for other FU Ori systems and rapidly accreting young stellar disks and find a 4000-6000 K wind can explain the observed line profiles. Fitting the progenitor spectrum, we find $M_* = 0.2 \ M_\odot$ and $\dot{M}_\mathrm{progenitor} = 7.8 \times 10^{-8} \ M_\odot \ \mathrm{yr}^{-1}$. Finally, we discuss HBC 722 relative to V960 Mon, another FU Ori object we have previously studied in detail.

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