arXiv subjects

David R. Soderblom

Publications and source records attributed to David R. Soderblom.

At least 19 recordsLinked to original sources

arXiv2025

First detection of ices in intermediate-mass young stellar objects beyond the Milky Way

Using NIRSpec on JWST, we studied a sample of 15 intermediate-mass (1.8-4.1 Msun) young stellar objects (YSOs) previously identified with MIRI photometry in the low-metallicity NGC 346 star-forming cluster in the Small Magellanic Cloud (SMC). All objects, observed in the 1.7-5.3 micron range, show strong hydrogen recombination lines in the Paschen, Brackett, Pfund, and Humphreys series, confirming their very young ages. The spectra of 11 YSOs show prominent absorption bands from the three most important ice species (H2O, CO2, CO), marking the first detection of these ices in intermediate-mass YSOs beyond our Galaxy. In three YSOs, water ice appears to be in crystalline form. In some objects, we also detect 13CO2 and OCS ices -- never before observed beyond the Milky Way (MW) -- and methanol ice in at least one star. We compared the column densities of H2O, CO2, and CO ices with those measured in more and less massive protostars in the MW and Large Magellanic Cloud, finding that in NGC 346 ice column densities reach values nearly an order of magnitude lower than in more massive objects (~1x10^{17} cm-2 for water and ~1x10^{16} cm-2 for CO2 and CO). However, the relative proportions of the ice species abundances do not differ from those in massive MW YSOs. This suggests that metallicity may not significantly affect ice chemistry in protoplanetary discs and that, shielded by the protostellar envelope or deep in the midplane, circumstellar material is likely impervious to the radiation environment.

astro-ph.GA↗

arXiv2024

Protoplanetary discs around sun-like stars appear to live longer when the metallicity is low

Previous Hubble Space Telescope (HST) observations of the star-forming cluster NGC 346 in the Small Magellanic Cloud (SMC) had revealed a large population of pre-main sequence (PMS) candidates, characterised by Halpha excess emission in their photometry. However, without access to spectroscopy, the nature of these objects remained unclear. Using the NIRSpec instrument on board JWST, we studied a sample of these stars, with masses in the range ~0.9-1.8 Msun, effective temperatures in the range 4,500-8,000 K, and PMS ages between ~0.1 and 30 Myr. Here we present the first spectra of solar-mass PMS stars in the metal-poor SMC (Z=1/8 Zsun) and discuss the physical properties of ten representative sources with good signal-to-noise ratio. The observations indicate that even the oldest of these PMS candidates are still accreting gas with typical rates of ~10^{-8} Msun/yr for stars older than ~10 Myr, confirming their PMS nature. The spectra also reveal near-infrared excess and molecular hydrogen excitation lines consistent with the presence of discs around these stars. These findings suggest that in a low-metallicity environment circumstellar discs can live longer than previously thought.

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arXiv2024

Young Stellar Objects in NGC 346: A JWST NIRCam/MIRI Imaging Survey

We present a JWST imaging survey with NIRCam and MIRI of NGC 346, the brightest star-forming region in the Small Magellanic Cloud (SMC). By combining aperture and point spread function (PSF) photometry of eleven wavelength bands across these two instruments, we have detected more than 200,000 unique sources. Using near-infrared (IR) color analysis, we observe various evolved and young populations, including 196 young stellar objects (YSOs) and pre-main sequence stars suitable for forthcoming spectroscopic studies. We expand upon this work, creating mid-IR color-magnitude diagrams and determining color cuts to identify 833 reddened sources which are YSO candidates. We observe that these candidate sources are spatially associated with regions of dusty, filamentary nebulosity. Furthermore, we fit model YSO spectral energy distributions (SEDs) to a selection of sources with detections across all of our MIRI bands. We classify with a high degree of confidence 23 YSOs in this sample and estimate their radii, bolometric temperatures, luminosities, and masses. We detect YSOs approaching 1 solar mass, the lowest-mass extragalactic YSOs confirmed to date.

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arXiv2023

A HST Study of the Substellar Population of NGC 2024

We performed a HST/WFC3-IR imaging survey of the young stellar cluster NGC 2024 in three filters probing the 1.4~$\mu$m H$_2$O absorption feature, characteristic of the population of low mass and sub-stellar mass objects down to a few Jupyter masses. We detect 812 point sources, 550 of them in all 3 filters with signal to noise greater than 5. Using a distance-independent two-color diagram we determine extinction values as high as $A_V\simeq 40$. We also find that the change of effective wavelengths in our filters results in higher $A_V$ values as the reddening increases. Reconstructing a dereddened color-magnitude diagram we derive a luminosity histogram for both the full sample of candidate cluster members and for an extinction-limited sub-sample containing the 50% of sources with $A_V\lesssim 15$. Assuming a standard extinction law like Cardelli et al. (1989) with a nominal $R_V$=3.1 we produce a luminosity function in good agreement with the one resulting from a Salpeter-like Initial Mass Function for a 1~Myr isochrone. There is some evidence of an excess of luminous stars in the most embedded region. We posit that the correlation may be due to those sources being younger, and therefore overluminous than the more evolved and less extinct cluster's stars. We compare our classification scheme based on the depth of the 1.4$\mu$m photometric feature with the results from the spectroscopic survey of Levine et al. (2006), and we report a few peculiar sources and morphological features typical of the rich phenomenology commonly encountered in young star-forming regions.

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arXiv2023

Asteroseismology with the Roman Galactic Bulge Time-Domain Survey

Asteroseismology has transformed stellar astrophysics. Red giant asteroseismology is a prime example, with oscillation periods and amplitudes that are readily detectable with time-domain space-based telescopes. These oscillations can be used to infer masses, ages and radii for large numbers of stars, providing unique constraints on stellar populations in our galaxy. The cadence, duration, and spatial resolution of the Roman galactic bulge time-domain survey (GBTDS) are well-suited for asteroseismology and will probe an important population not studied by prior missions. We identify photometric precision as a key requirement for realizing the potential of asteroseismology with Roman. A precision of 1 mmag per 15-min cadence or better for saturated stars will enable detections of the populous red clump star population in the Galactic bulge. If the survey efficiency is better than expected, we argue for repeat observations of the same fields to improve photometric precision, or covering additional fields to expand the stellar population reach if the photometric precision for saturated stars is better than 1 mmag. Asteroseismology is relatively insensitive to the timing of the observations during the mission, and the prime red clump targets can be observed in a single 70 day campaign in any given field. Complementary stellar characterization, particularly astrometry tied to the Gaia system, will also dramatically expand the diagnostic power of asteroseismology. We also highlight synergies to Roman GBTDS exoplanet science using transits and microlensing.

astro-ph.IM↗

arXiv2023

Discovery of dusty sub-solar mass young stellar objects in NGC 346 with JWST/NIRCam

JWST observations of NGC 346, a star-forming region in the metal-poor Small Magellanic Cloud, reveal a substantial population of sub-solar mass young stellar objects (YSOs) with IR excess. We detected $\sim$500 YSOs and pre main sequence (PMS) stars from more than 45,000 unique sources utilizing all four NIRCam wide filters with deep, high-resolution imaging, where ongoing low-mass star formation is concentrated along dust filaments. From these observations, we construct detailed near-IR colour-magnitude diagrams with which preliminary categorizations of YSO classes are made. For the youngest, most deeply-embedded objects, JWST/NIRCam reaches over 10 magnitudes below Spitzer observations at comparable wavelengths, and two magnitudes fainter than HST for more-evolved PMS sources, corresponding to $\sim$0.1 M$_\odot$. For the first time in an extragalactic environment, we detect embedded low-mass star-formation. Furthermore, evidence of IR excess and accretion suggests that dust required for rocky planet formation is present at metallicities as low as 0.2 $Z_\odot$.

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arXiv2020

HST astrometry in the Orion Nebula Cluster: census of low-mass runaways

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the magnitude range of $14.2<m_{\rm F775W}<24.7$, thus probing the stellar masses of the ONC from $\sim$0.4 $M_\odot$ down to $\sim$0.02 $M_\odot$ over an area of $\sim$550 arcmin$^2$. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on the stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at $σ_{v,x}=0.94$ and $σ_{v,y}=1.25$ mas yr$^{-1}$) might be biased by the overlapping young stellar populations of Orion A. We identified 4 new ONC candidate runaways based on HST and the Gaia DR2 data, all with masses less than $\sim$1 $M_\odot$. The total census of known candidate runaway sources is 10 -- one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them has the tangential velocity exceeding 20 km s$^{-1}$. If most of them indeed originated in the ONC, it may compel re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.

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arXiv2016

Asteroseismology of the Hyades with K2: first detection of main-sequence solar-like oscillations in an open cluster

The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2 mission, and short-cadence data were collected on a number of cool main-sequence stars. Here, we report results on two F-type stars that show detectable oscillations of a quality that allows asteroseismic analyses to be performed. These are the first ever detections of solar-like oscillations in main-sequence stars in an open cluster.

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arXiv2014

Ages of stars: Methods and uncertainties

Estimating ages for stars is difficult at best, but Galactic problems have their own requirements that go beyond those for other areas of astrophysics. As in other areas, asteroseismology is helping, and in this review I discuss some of the general problems encountered and some specific to large-scale studies of the Milky Way.

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arXiv2014

George Howard Herbig, 1920-2013

A great astronomer, George Herbig, passed away in Honolulu on October 12, 2013, at the age of 93. His life and career were long and productive, and consistently dedicated to the careful, thorough research that earned him his reputation. Herbig spent most of his career at Lick Observatory, first as a graduate student, and then from 1949 to 1987 as a staff member, rising through the ranks to Astronomer and becoming Professor as well when the observatory moved to the University of California at Santa Cruz in the late 1960s. Herbig retired from UCSC in 1987 and spent the remainder of his life in Honolulu at the Institute for Astronomy of the University of Hawai`i.

physics.hist-ph↗

arXiv2013

Ages of young stars

Determining the sequence of events in the formation of stars and planetary systems and their time-scales is essential for understanding those processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the age challenge for young stars, specifically those less than ~100 Myr old. Most age determination methods that we discuss are primarily applicable to groups of stars but can be used to estimate the age of individual objects. A reliable age scale is established above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young clusters, and consistency is shown between these ages and those from the upper main sequence and the main sequence turn-off -- if modest core convection and rotation is included in the models of higher-mass stars. Other available methods for age estimation include the kinematics of young groups, placing stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface gravity measurement, rotation and activity, and lithium abundance. We review each of these methods and present known strengths and weaknesses. Below ~20 Myr, both model-dependent and observational uncertainties grow, the situation is confused by the possibility of age spreads, and no reliable absolute ages yet exist. The lack of absolute age calibration below 20 Myr should be borne in mind when considering the lifetimes of protostellar phases and circumstellar material.

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arXiv2010

A Multi-color Optical Survey of the Orion Nebula Cluster. II. the H-R diagram

We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study the color-color diagrams in BVI, plus a narrow-band filter centered at 6200A, finding evidences that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC, while a better agreement is found employing synthetic intrinsic colors obtained constraining the typical lower surface gravity of young stars. We refine these model colors even further, empirically, by comparison with a sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and add 65 newly classified stars from slit spectroscopy and 182 M-type from narrow-band photometry; in this way we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive reddening and accretion excess from the location of each star in the BVI color-color diagram. This enables us to accurately determine the extinction of the ONC members. We adopt a lower distance for the ONC than previously assumed, based on recent parallax measurements. With a careful choice also of the spectral type-temperature transformation, we produce the new H-R diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a lower luminosity for early types. We determine the age distribution of the population, peaking at 2-3 Myr, a higher age than previously estimated. We study the distribution of the members in the mass-age plane, and find that taking into account selection effects due to incompleteness removes an apparent correlation between mass and age. We derive the IMF for low- and intermediate-mass members of the ONC, which turns out to be model-dependent, and shows a turn-over at ~<0.2Msun.

astro-ph.GA↗

arXiv2010

The Ages of Stars

The age of an individual star cannot be measured, only estimated through mostly model-dependent or empirical methods, and no single method works well for a broad range of stellar types or for a full range in age. This review presents a summary of the available techniques for age-dating stars and ensembles of stars, their realms of applicability, and their strengths and weaknesses. My emphasis is on low-mass stars because they are present from all epochs of star formation in the Galaxy and because they present both special opportunities and problems. The ages of open clusters are important for understanding the limitations of stellar models and for calibrating empirical age indicators. For individual stars, a hierarchy of quality for the available age-dating methods is described. Although our present ability to determine the ages of even the nearest stars is mediocre, the next few years hold great promise as asteroseismology probes beyond stellar surfaces and starts to provide precise interior properties of stars and as models continue to improve when stressed by better observations.

astro-ph.SR↗

arXiv2009

The Metallicity of the Pleiades

We have measured the abundances of Fe, Si, Ni, Ti, and Na in 20 Pleiads with \teff values near solar and with low \vsini using high-resolution, high signal-to-noise echelle spectra. We have validated our procedures by also analyzing 10 field stars of a range of temperatures and metallicities that were observed by \citet{Vale05}. Our result for the Pleiades is [Fe/H] = $+0.03\pm0.02\pm0.05$ (statistical and systematic). The average of published measurements for the Pleiades is $+0.042\pm0.021$.

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arXiv2009

A Multi-Color Optical Survey of the Orion Nebula Cluster. Part I: the Catalog

We present U, B, V, I broad-band, 6200A TiO medium-band and Halpha photometry of the Orion Nebula Cluster obtained with the WFI imager at the ESO/MPI 2.2 telescope. The nearly-simultaneous observations cover the entire ONC in a field of about 34x34 arcmin. They enable us to determine stellar colors avoiding the additional scatter in the photometry induced by stellar variability typical of pre-main sequence stars. We identify 2,612 point-like sources in I band, 58%, 43% and 17% of them detected also in V, B and U, respectively. 1040 sources are identified in Halpha band. In this paper we present the observations, the calibration techniques, and the resulting catalog. We show the derived CMD of the population and discuss the completeness of our photometry. We define a spectro-photometric TiO index from the fluxes in V, I, and TiO-band. We find a correlation between the index and the spectral type valid for M-type stars, that is accurate to better than 1 spectral sub-class for M3-M6 types and better than 2 spectral subclasses for M0-M2 types. This allows us to newly classify 217 stars. We subtract from our Halpha photometry the photospheric continuum at its wavelength, deriving calibrated line excess for the full sample. This represents the largest Halpha star catalog obtained to date on the ONC. This data set enables a full re-analysis of the properties of the Pre-Main Sequence population in the Orion Nebula Cluster to be presented, in an accompanying paper.

astro-ph.GA↗

arXiv2009

The Metallicity of the HD 98800 System

Pre-main sequence (PMS) binaries and multiples enable critical tests of stellar models if masses, metallicities, and luminosities of the component stars are known. We have analyzed high-resolution, high signal-to-noise echelle spectra of the quadruple-star system HD 98800 and using spectrum synthesis, computed fits to the composite spectrum for a full range of plausible stellar parameters for the components. We consistently find that sub-solar metallicity yields fits with lower $χ^2$ values, with an overall best-fit of $[M/H] = -0.20\pm0.10$. This metallicity appears to be consistent with PMS evolutionary tracks for the measured masses and luminosities of the components of HD 98800 but additional constraints on the system and modelling are needed.

astro-ph.SR↗

arXiv2008

The HST/ACS Atlas of Protoplanetary Disks in the Great Orion Nebula

We present the atlas of protoplanetary disks in the Orion Nebula based on the ACS/WFC images obtained for the HST Treasury Program on the Orion Nebula Cluster. The observations have been carried out in 5 photometric filters nearly equivalent to the standard B, V, Halpha, I, and z passbands. Our master catalog lists 178 externally ionized proto-planetary disks (proplyds), 28 disks seen only in absorption against the bright nebular background (silhouette disks), 8 disks seen only as dark lanes at the midplane of extended polar emission (bipolar nebulae or reflection nebulae) and 5 sources showing jet emission with no evidence of neither external ionized gas emission nor dark silhouette disks. Many of these disks are associated with jets seen in Halpha and circumstellar material detected through reflection emission in our broad-band filters; approximately 2/3 have identified counterparts in x-rays. A total of 47 objects (29 proplyds, 7 silhouette disks, 6 bipolar nebulae, 5 jets with no evidence of proplyd emission or silhouette disk) are new detections with HST. We include in our list 4 objects previously reported as circumstellar disks which have not been detected in our HST/ACS images either because they are hidden by the bleeding trails of a nearby saturated bright star or because of their location out of the HST/ACS Treasury Program field. Other 31 sources previously reported as extended objects do not harbor a stellar source in our HST/ACS images. We also report on the detection of 16 red, elongated sources. Their location at the edges of the field, far from the Trapezium Cluster core (> 10'), suggests that these are probably background galaxies observed through low extinction regions of the Orion Molecular Cloud OMC-1.

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arXiv2007

Keck Hires Spectroscopy of Candidate Post T Tauri Stars

We use high-signal-to-noise ($\sim$150-450), high resolution ($R{\sim}45,000$) Keck HIRES spectroscopy of 13 candidate post T Tauri stars to derive basic physical parameters, lithium abundances and radial velocities. We place our stars in the M$_v$-T$_{eff}$ plane for use in determining approximate ages from pre-main sequence isochrones, and confirm these using three relative age indicators in our analysis: Li abundances, chromospheric emission and the kinematic {\it U-V} plane. Using the three age criteria we identify 5 stars (HIP 54529, HIP 62758, HIP 63322, HIP 74045, and HIP 104864) as probable post T Tauri stars with ages between 10 and 100 Myr. We confirm HIP 54529 as an SB2 and HIP 63322 as an SB1 star. We also examine irregular photometric variability of PTTs using the {\it HIPPARCOS} photometry annex. Two of our PTT stars exhibit near-IR excesses compared to Kurucz model flux; while recent work suggests classical T Tauri stars evince similar {\it JHK} excesses presumably indicative of non-photospheric (disk) emission, our results may be illusory artifacts of the chosen {\it I}-band normalization. Near-IR excesses we see in a literature-based sample of PTTs appear to be artifacts of previous spectral type-based $T_{\rm eff}$ values. Indeed, comparison of the homology of their observed and model photospheric SED's suggests that photometric temperatures are more reliable than temperatures based on spectral standards for the cooler temperature ranges of the stars in this sample. We conclude that our age oriented analysis is a robust means to select samples of nearby, young, isolated post T Tauri stars that otherwise masquerade as normal field stars.

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