15 research outputs found
Are There Age Spreads in Star Forming Regions?
A luminosity spread at a given effective temperature is ubiquitously seen in
the Hertzsprung-Russell (HR) diagrams of young star forming regions and often
interpreted in terms of a prolonged period (>=10 Myr) of star formation. I
review the evidence that the observed luminosity spreads are genuine and not
caused by astrophysical sources of scatter. I then address whether the
luminosity spreads necessarily imply large age spreads, by comparing HR diagram
ages with ages from independent clocks such as stellar rotation rate, the
presence of circumstellar material and lithium depletion. I argue that whilst
there probably is a true luminosity dispersion, there is little evidence to
support age spreads larger than a few Myr. This paradox could be resolved by
brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of
Large Surveys, 8 page
Photo-reverberation mapping of a protoplanetary accretion disk around a T Tauri star
This is the final version. Available from the American Astronomical Society via the DOI in this recordTheoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall" at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1 au scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the ρ Ophiuchi star-forming region, by detecting the light-travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 μm) and K (2.2 μm) bands were synchronized while the 4.5 μm emission lagged by 74.5 ± 3.2 s. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 au from the protostar on average, with an error of order 0.01 au. This size is likely larger than the range of magnetospheric truncations and consistent with an optically and geometrically thick disk front at the dust sublimation radius at ∼1500 K. The widths of the cross-correlation functions between the data in different wavebands place possible new constraints on the geometry of the disk.National Science Foundation (NSF
Spatially Resolved Magnetic Field Structure in the Disk of a T Tauri Star
Magnetic fields in accretion disks play a dominant role during the star
formation process but have hitherto been observationally poorly constrained.
Field strengths have been inferred on T Tauri stars themselves and possibly in
the innermost part of the accretion disk, but the strength and morphology of
the field in the bulk of the disk have not been observed. Unresolved
measurements of polarized emission (arising from elongated dust grains aligned
perpendicular to the field) imply average fields aligned with the disks.
Theoretically, the fields are expected to be largely toroidal, poloidal, or a
mixture of the two, which imply different mechanisms for transporting angular
momentum in the disks of actively accreting young stars such as HL Tau. Here we
report resolved measurements of the polarized 1.25 mm continuum emission from
HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the
major axis (~210 AU diameter) of the disk. From this we conclude that the
magnetic field inside the disk at this scale cannot be dominated by a vertical
component, though a purely toroidal field does not fit the data well either.
The unexpected morphology suggests that the magnetic field's role for the
accretion of a T Tauri star is more complex than the current theoretical
understanding.Comment: Accepted for publication in Natur
Accreting Protoplanets in the LkCa 15 Transition Disk
Exoplanet detections have revolutionized astronomy, offering new insights
into solar system architecture and planet demographics. While nearly 1900
exoplanets have now been discovered and confirmed, none are still in the
process of formation. Transition discs, protoplanetary disks with inner
clearings best explained by the influence of accreting planets, are natural
laboratories for the study of planet formation. Some transition discs show
evidence for the presence of young planets in the form of disc asymmetries or
infrared sources detected within their clearings, as in the case of LkCa 15.
Attempts to observe directly signatures of accretion onto protoplanets have
hitherto proven unsuccessful. Here we report adaptive optics observations of
LkCa 15 that probe within the disc clearing. With accurate source positions
over multiple epochs spanning 2009 - 2015, we infer the presence of multiple
companions on Keplerian orbits. We directly detect H{\alpha} emission from the
innermost companion, LkCa 15 b, evincing hot (~10,000 K) gas falling deep into
the potential well of an accreting protoplanet.Comment: 35 pages, 3 figures, 1 table, 9 extended data item
Do All Stars in the Solar Neighbourhood Form in Clusters?
We present a global study of low mass, young stellar object (YSO) surface
densities (Sigma) in nearby (< 500 pc) star forming regions based on a
comprehensive collection of Spitzer Space Telescope surveys. We show that the
distribution of YSO surface densities is a smooth distribution, being
adequately described by a lognormal function from a few to 10^3 YSOs per pc^2,
with a peak at ~22 stars pc^-2. The observed lognormal Sigma is consistent with
predictions of hierarchically structured star-formation at scales below 10 pc,
arising from the molecular cloud structures. We do not find evidence for
multiple discrete modes of star-formation (e.g. clustered and distributed).
Comparing the observed Sigma distribution to previous Sigma threshold
definitions of clusters show that they are arbitrary. We find that only a low
fraction (< 26$) of stars are formed in dense environments where their
formation/evolution (along with their circumstellar disks and/or planets) may
be affected by the close proximity of their low-mass neighbours.Comment: 7 Pages, 2 Figures, JENAM conference (Lisbon
Low-mass and sub-stellar eclipsing binaries in stellar clusters
We highlight the importance of eclipsing double-line binaries in our
understanding on star formation and evolution. We review the recent discoveries
of low-mass and sub-stellar eclipsing binaries belonging to star-forming
regions, open clusters, and globular clusters identified by ground-based
surveys and space missions with high-resolution spectroscopic follow-up. These
discoveries provide benchmark systems with known distances, metallicities, and
ages to calibrate masses and radii predicted by state-of-the-art evolutionary
models to a few percent. We report their density and discuss current
limitations on the accuracy of the physical parameters. We discuss future
opportunities and highlight future guidelines to fill gaps in age and
metallicity to improve further our knowledge of low-mass stars and brown
dwarfs.Comment: 30 pages, 5 figures, no table. Review pape
New Pleiades Eclipsing Binaries and a Hyades Transiting System Identified by K2
We present the discovery in Kepler's K2 mission observations and our follow-up radial velocity (RV) observations from Keck/HIRES for four eclipsing binary (EB) star systems in the young benchmark Pleiades and Hyades clusters. Based on our modeling results, we announce two new low mass ( ) EBs among Pleiades members (HCG 76 and MHO 9) and we report on two previously known Pleiades binaries that are also found to be EB systems (HII 2407 and HD 23642). We measured the masses of the binary HCG 76 to lesssim2.5% precision, and the radii to lesssim4.5% precision, which together with the precise effective temperatures yield an independent Pleiades distance of 132 ± 5 pc. We discuss another EB toward the Pleiades that is a possible but unlikely Pleiades cluster member (AK II 465). The two new confirmed Pleiades systems extend the mass range of Pleiades EB components to 0.2–2 . Our initial measurements of the fundamental stellar parameters for the Pleiades EBs are discussed in the context of the current stellar models and the nominal cluster isochrone, finding good agreement with the stellar models of Baraffe et al. at the nominal Pleiades age of 120 Myr. Finally, in the Hyades, we report a new low mass eclipsing system (vA 50) that was concurrently discovered and studied by Mann et al. We confirm that the eclipse is likely caused by a Neptune-sized transiting planet, and with the additional RV constraints presented here we improve the constraint on the maximum mass of the planet to be lesssim1.2 MJup
New Pleiades Eclipsing Binaries and a Hyades Transiting System Identified by K2
We present the discovery in Kepler's K2 mission observations and our follow-up radial velocity (RV) observations from Keck/HIRES for four eclipsing binary (EB) star systems in the young benchmark Pleiades and Hyades clusters. Based on our modeling results, we announce two new low mass ( ) EBs among Pleiades members (HCG 76 and MHO 9) and we report on two previously known Pleiades binaries that are also found to be EB systems (HII 2407 and HD 23642). We measured the masses of the binary HCG 76 to lesssim2.5% precision, and the radii to lesssim4.5% precision, which together with the precise effective temperatures yield an independent Pleiades distance of 132 ± 5 pc. We discuss another EB toward the Pleiades that is a possible but unlikely Pleiades cluster member (AK II 465). The two new confirmed Pleiades systems extend the mass range of Pleiades EB components to 0.2–2 . Our initial measurements of the fundamental stellar parameters for the Pleiades EBs are discussed in the context of the current stellar models and the nominal cluster isochrone, finding good agreement with the stellar models of Baraffe et al. at the nominal Pleiades age of 120 Myr. Finally, in the Hyades, we report a new low mass eclipsing system (vA 50) that was concurrently discovered and studied by Mann et al. We confirm that the eclipse is likely caused by a Neptune-sized transiting planet, and with the additional RV constraints presented here we improve the constraint on the maximum mass of the planet to be lesssim1.2 MJup
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Circumstellar Disk Accretion Across the Lagoon Nebula: The Influence of Environment and Stellar Mass
Abstract
Pre-main-sequence disk accretion is pivotal for determining the final stellar properties and the early conditions for close-in planets. We aim to establish the impact of internal (stellar mass) and external (radiation field) parameters on the disk evolution in the Lagoon Nebula massive star-forming region. We employ simultaneous u, g, r, i, Hα time-series photometry, archival infrared data, and high-precision K2 light curves to derive the stellar, disk, and accretion properties for 1012 Lagoon Nebula members. We estimate that of all young stars in the Lagoon Nebula, 34%–37% have inner disks traceable down to ∼12 μm, while 38%–41% are actively accreting. We detect disks ∼1.5 times more frequently around G, K, and M stars than around higher-mass stars, which appear to deplete their inner disks on shorter timescales. We find tentative evidence for a faster disk evolution in the central regions of the Lagoon Nebula, where the bulk of the O/B population is located. Conversely, disks appear to last longer at the nebula outskirts, where the measured fraction of disk-bearing stars tends to exceed that of accreting and disk-free stars. The derived mass accretion rates show a nonuniform dependence on stellar mass between ∼0.2 and 5 M
⊙. In addition, the typical accretion rates appear to differ across the Lagoon Nebula extension, with values twice lower in the core region than at its periphery. Finally, we detect tentative radial density gradients in the surface accretion shocks, leading to lags in the appearance of light curve brightness features as a function of wavelength that can amount to ∼7%–30% of the rotation period.</jats:p
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A Warm Jupiter-sized Planet Transiting the Pre-main-sequence Star V1298 Tau
We report the detection of V1298 Tau b, a warm Jupiter-sized planet (R P = 0.91 ± 0.05 R Jup, P = 24.1 days) transiting a young solar analog with an estimated age of 23 Myr. The star and its planet belong to Group 29, a young association in the foreground of the Taurus-Auriga star-forming region. While hot Jupiters have been previously reported around young stars, those planets are non-transiting and near-term atmospheric characterization is not feasible. The V1298 Tau system is a compelling target for follow-up study through transmission spectroscopy and Doppler tomography owing to the transit depth (0.5%), host star brightness (K s = 8.1 mag), and rapid stellar rotation (= 23 km s-1). Although the planet is Jupiter-sized, its mass is currently unknown due to high-amplitude radial velocity jitter. Nevertheless, V1298 Tau b may help constrain formation scenarios for at least one class of close-in exoplanets, providing a window into the nascent evolution of planetary interiors and atmospheres