arXiv subjects

Lynne A. Hillenbrand

Publications and source records attributed to Lynne A. Hillenbrand.

At least 37 records · Page 2Linked to original sources

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

Ages of Stars and Planets in the Kepler Field Younger Than Four Billion Years

Recent analyses of FGK stars in open clusters have helped clarify the precision with which a star's rotation rate and lithium content can be used as empirical indicators for its age. Here we apply this knowledge to stars observed by Kepler. Rotation periods are drawn from previous work; lithium is measured from new and archival Keck/HIRES spectra. We report rotation-based ages for 23,813 stars (harboring 795 known planets) for which our method is applicable. We find that our rotational ages recover the ages of stars in open clusters spanning 0.04-2.5 Gyr; they also agree with over 90% of the independent lithium ages. The resulting yield includes 63 planets younger than 1 Gyr at 2$σ$, and 109 with median ages below 1 Gyr. This is about half the number expected under the classic assumption of a uniform star formation history. The age distribution that we observe, rather than being uniform, shows that the youngest stars in the Kepler field are 3-5 times rarer than stars 3 Gyr old. This trend holds for both known planet hosts and for the parent stellar sample. We attribute this "demographic cliff" to a combination of kinematic heating and a declining star formation rate in the Galaxy's thin disk, and highlight its impact on the age distribution of known transiting exoplanets.

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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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arXiv2024

The ZTF Source Classification Project: III. A Catalog of Variable Sources

The classification of variable objects provides insight into a wide variety of astrophysics ranging from stellar interiors to galactic nuclei. The Zwicky Transient Facility (ZTF) provides time series observations that record the variability of more than a billion sources. The scale of these data necessitates automated approaches to make a thorough analysis. Building on previous work, this paper reports the results of the ZTF Source Classification Project (SCoPe), which trains neural network and XGBoost machine learning (ML) algorithms to perform dichotomous classification of variable ZTF sources using a manually constructed training set containing 170,632 light curves. We find that several classifiers achieve high precision and recall scores, suggesting the reliability of their predictions for 209,991,147 light curves across 77 ZTF fields. We also identify the most important features for XGB classification and compare the performance of the two ML algorithms, finding a pattern of higher precision among XGB classifiers. The resulting classification catalog is available to the public, and the software developed for SCoPe is open-source and adaptable to future time-domain surveys.

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arXiv2024

The Effect of Starspots on Spectroscopic Age and Mass Estimates of Non-Accreting T~Tauri Stars in the Taurus-Auriga Star Forming Region

Accurate age and mass determinations for young pre-main sequence stars are made challenging by the presence of large-scale starspots. We present results from a near-infrared spectroscopic survey of ten T-Tauri Stars in Taurus-Auriga that characterize spot filling factors and temperatures, the resulting effects on temperature and luminosity determinations, and the consequences for inferred stellar masses and ages. We constructed composite models of spotted stars by combining BTSettl-CIFIST synthetic spectra of atmospheres to represent the spots and the photosphere along with continuum emission from a warm inner disk. Using a Markov-Chain Monte-Carlo algorithm, we find the best-fit spot and photospheric temperatures, spot filling factors, as well as disk filling factors. This methodology allowed us to reproduce the 0.75-2.40 micron stellar spectra and molecular feature strengths for all of our targets, disentangling the complicated multi-component emission. For a subset of stars with multi-epoch observations spanning an entire stellar rotation, we correlate the spectral variability and changes in the filling factors with rotational periods observed in K2 and AAVSO photometry. Combining spot-corrected effective temperatures and Gaia distances, we calculate luminosities and use the Stellar Parameters of Tracks with Starspots (SPOTS) models to infer spot-corrected masses and ages for our sample of stars. Our method of accounting for spots results in an average increase of 60% in mass and a doubling in age with respect to traditional methods using optical spectra that do not account for the effect of spots.

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arXiv2024

The Emerging Stellar Complex in Mon R2: Membership and Optical Variability Classification

Monoceros R2 (Mon R2) is one of the closest large active star-forming regions. This extremely young and partially embedded region provides an excellent laboratory for studying star formation and the early evolution of young stellar objects (YSOs). In this paper, we conduct an optical study of the greater Mon R2 region. Beginning with 1690 previously identified candidate YSOs, we used 496 sources with good proper motions and parallaxes from Gaia DR3 to determine the astrometric properties for likely members of Mon R2. We then used both astrometric and photometric (isochronal and variability) criteria to determine that 308 of these stars are highly probable members. Using the same criteria, we considered a broad area search around Mon R2 in Gaia DR3, and separated candidate members from field stars. In total, we selected 651 likely new cluster members that had been missed in the previous x-ray and infrared excess selection techniques used in the past to establish cluster membership. Revised astrometric properties of the cluster were found using the combined sample of ~959 highly probable member stars. For the literature plus new candidate member list, optical light curves were compiled from the Zwicky Transient Facility. For 470 identified variable sources, we attempted classification based on the Flux Asymmetry (M) and Quasi-Periodicity (Q) metrics. We find that Mon R2 is dominated by quasi-periodic symmetric variables, with aperiodic sources also a significant population. A few tens of large-amplitude variables are identified that may be of interest for further study.

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arXiv2024

New insights on the accretion properties of Class 0 protostars from 2 micron spectroscopy

Sun-like stars are thought to accrete most of their final mass during the protostellar phase, during which the stellar embryo is surrounded by an infalling dense envelope. We present an analysis of 26 $K$-band spectra of Class 0 protostars, which are the youngest protostars. 18 of these are new observations made with the Keck MOSFIRE instrument. HI Br$γ$ , several H$_2$, and CO $Δ\,v\;=\;2$ features are detected and analyzed. We detect Br$γ$ emission in 62%, CO overtone emission in 50%, and H$_2$ emission in 90% of sources. The HI and CO emission is associated with accretion while the H$_2$ lines are consistent with shock excitation indicating jets/outflows. Six objects exhibit photospheric absorption features, with almost no outflow activity, and no detection of the accretion-related Br$γ$ emission line. Comparing these results with archival sample of Class I $K$-band spectra, we find that the CO and Br$γ$ emission lines are systematically more luminous in Class 0s, suggesting the accretion is on average more vigorous in the Class 0 phase. Typically associated with the heated inner accretion disk, the much higher detection rate of CO overtone emission in Class 0s indicate also that episodes of high accretion activity are more frequent in Class 0 systems. The kinematics of the Class 0 CO overtone emission suggest either an accretion-heated inner disk, or material directly infalling onto the central region. This could point toward an accretion mechanism of different nature in Class 0 systems than the typical picture of magnetospheric accretion.

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arXiv2024

The 2022-2023 accretion outburst of the young star V1741 Sgr

V1741 Sgr (= SPICY 71482/Gaia22dtk) is a Classical T Tauri star on the outskirts of the Lagoon Nebula. After at least a decade of stability, in mid-2022, the optical source brightened by ~3 mag over two months, remained bright until early 2023, then dimmed erratically over the next four months. This event was monitored with optical and infrared spectroscopy and photometry. Spectra from the peak (October 2022) indicate an EX Lup-type (EXor) accretion outburst, with strong emission from H I, He I, and Ca II lines and CO bands. At this stage, spectroscopic absorption features indicated a temperature of T ~ 4750 K with low-gravity lines (e.g., Ba II and Sr II). By April 2023, with the outburst beginning to dim, strong TiO absorption appeared, indicating a cooler T ~ 3600 K temperature. However, once the source had returned to its pre-outburst flux in August 2023, the TiO absorption and the CO emission disappeared. When the star went into outburst, the source's spectral energy distribution became flatter, leading to bluer colours at wavelengths shorter than ~1.6 microns and redder colours at longer wavelengths. The brightening requires a continuum emitting area larger than the stellar surface, likely from optically thick circumstellar gas with cooler surface layers producing the absorption features. Additional contributions to the outburst spectrum may include blue excess from hotspots on the stellar surface, emission lines from diffuse gas, and reprocessed emission from the dust disc. Cooling of the circumstellar gas would explain the appearance of TiO, which subsequently disappeared once this gas had faded and the stellar spectrum reemerged.

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arXiv2024

ABYSS II: Identification of young stars in optical SDSS spectra and their properties

We developed a tool that measures equivalent widths of various lines in low resolution optical spectra, and it was applied to stellar spectra obtained as part of SDSS-V and LAMOST programs. These lines, such as Li I which directly indicates stellar youth, or optical H I and Ca II which in emission indicate activity associated with stellar youth, are commonly seen in YSOs. We observe several notable differences in the properties of these lines between YSOs and the field stars. Using these data, we devise a set of criteria through which it is possible to confirm the youth of stars that have been observed by the ABYSS program, as well as to identify likely young stars that have serendipitously been observed by other programs. We examine the decrement of H lines seen in emission in CTTSs, and estimate the properties of the accretion stream that is responsible for the production of these lines. Finally, we examine the evolution of Li I as a function of age, and characterize the scatter in its abundance that appears to be intrinsic in young M dwarfs.

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arXiv2023

Transient Corotating Clumps Around Adolescent Low-Mass Stars From Four Years of TESS

Complex periodic variables (CPVs) are stars that exhibit highly structured and periodic optical light curves. Previous studies have indicated that these stars are typically disk-free pre-main-sequence M dwarfs with rotation periods ranging from 0.2 to 2 days. To advance our understanding of these enigmatic objects, we conducted a blind search using TESS 2-minute data of 65,760 K and M dwarfs with $T$<16 mag and $d$<150 pc. We found 50 high-quality CPVs, and subsequently determined that most are members of stellar associations. Among the new discoveries are the brightest ($T$$\approx$9.5 mag), closest ($d$$\approx$20 pc), and oldest ($\approx$200 Myr) CPVs known. One exceptional object, LP 12-502, exhibited up to eight flux dips per cycle. Some of these dips coexisted with slightly different periods, and the shortest-duration dips precisely matched the expected timescale for transiting small bodies at the corotation radius. Broadly, our search confirms that CPVs are mostly young ($\lesssim$150 Myr) and low-mass ($\lesssim$0.4 $M_\odot$). The flux dips characteristic of the class have lifetimes of $\approx$100 cycles, although stellar flares seem to induce sudden dip collapse once every few months. The most plausible explanation for these phenomena remains corotating concentrations of gas or dust. The gas or dust is probably entrained by the star's magnetic field, and the sharp features could result from a multipolar field topology, a hypothesis supported by correspondences between the light curves of CPVs and of rapidly rotating B stars known to have multipolar magnetic fields.

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arXiv2023

Disk Cooling and Wind Lines As Seen In the Spectral Line Evolution of V960 Mon

We follow up our photometric study of the post-outburst evolution of the FU Ori object V960 Mon with a complementary spectroscopic study at high dispersion that uses time series spectra from Keck/HIRES. Consistent with the photometric results reported in Carvalho et al. 2023, we find that the spectral evolution of V960 Mon corresponds to a decrease in the temperature of the inner disk, driven by a combination of decreasing accretion rate and increasing inner disk radius. We also find that although the majority of the absorption lines are well-matched by our accretion disk model spectrum, there are several strong absorption line families and a few emission lines that are not captured by the model. By subtracting the accretion disk model from the data at each epoch, we isolate the wind/outflow components of the system. The residuals show both broad and highly blueshifted profiles, as well as narrow and only slightly blueshifted profiles, with some lines displaying both types of features.

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arXiv2023

Accretion Variability as a Guide to Stellar Mass Assembly

Variable accretion in young stellar objects reveals itself photometrically and spectroscopically over a continuum of timescales and amplitudes. Most dramatic are the large outbursts (e.g., FU Ori, V1647 Ori, and EX Lup type events), but more frequent are the less coherent, smaller burst-like variations in accretion rate. Improving our understanding of time-variable accretion directly addresses the fundamental question of how stars gain their masses. We review variability phenomena, as characterized from observations across the wavelength spectrum, and how those observations probe underlying physical conditions. The diversity of observed lightcurves and spectra at optical and infrared wavelengths defies a simple classification of outbursts and bursts into well-defined categories. Mid-infrared and submillimeter wavelengths are sensitive to lower-temperature phenomena and more embedded, younger sources, and it is currently unclear if observed flux variations probe similar or distinct physics relative to the shorter wavelengths. We highlight unresolved issues and emphasize the value of spectroscopy, multiwavelength studies, and ultimately patience in using variable accretion to understand stellar mass assembly.

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arXiv2023

RNO 54: A Previously Unappreciated FU Ori Star

We present evidence in support of the hypothesis that the young stellar object RNO 54 is a mature-stage FU Ori type source. The star was first cataloged as a ``red nebulous object" in the 1980s but appears to have undergone its outburst prior to the 1890s. Present-day optical and near-infrared spectra are consistent with those of other FU Ori type stars, both in the details of spectral line presence and shape, and in the overall change in spectral type from an FGK-type in the optical, to the M-type presented in the near-infrared. In addition, the spectral energy distribution of RNO 54 is well-fit by a pure-accretion disk model with parameters: $\dot{M} = 10^{-3.45\pm0.06}$ $M_\odot$ yr$^{-1}$, $M_* = 0.23\pm0.06 \ M_\odot$, and $R_\mathrm{inner} = 3.68\pm0.76 \ R_\odot$, though we believe $R_\mathrm{inner}$ is likely close to its upper range of $4.5 R_\odot$ in order to produce a $T_\mathrm{max} = 7000$ K that is consistent with the optical to near-infrared spectra. The resulting $L_\mathrm{acc}$ is $\sim 265 \ L_\odot$. To find these values, we adopted a source distance $d=1400$ pc and extinction $A_V=3.9$ mag, along with disk inclination $i=50$ deg based on consideration of confidence intervals from our initial disk model, and in agreement with observational constraints. The new appreciation of a well-known source as an FU Ori type object suggests that other such examples may be lurking in extant samples.

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arXiv2023

Twenty-Five Years of Accretion onto the Classical T Tauri Star TW Hya

Accretion plays a central role in the physics that governs the evolution and dispersal of protoplanetary disks. The primary goal of this paper is to analyze the stability over time of the mass accretion rate onto TW Hya, the nearest accreting solar-mass young star. We measure veiling across the optical spectrum in 1169 archival high-resolution spectra of TW Hya, obtained from 1998--2022. The veiling is then converted to accretion rate using 26 flux-calibrated spectra that cover the Balmer jump. The accretion rate measured from the excess continuum has an average of $2.51\times10^{-9}$~M$_\odot$~yr$^{-1}$ and a Gaussian distribution with a FWHM of 0.22 dex. This accretion rate may be underestimated by a factor of up to 1.5 because of uncertainty in the bolometric correction and another factor of 1.7 because of excluding the fraction of accretion energy that escapes in lines, especially Ly$α$. The accretion luminosities are well correlated with He line luminosities but poorly correlated with H$α$ and H$β$ luminosity. The accretion rate is always flickering over hours but on longer timescales has been stable over 25 years. This level of variability is consistent with previous measurements for most, but not all, accreting young stars.

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