Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations

Hoadley, Keri; Arulanantham, Nicole; Loyd, R. O. Parke; Kruczek, Nicholas

doi:10.3847/1538-4357/aa7fc1

Cited by 8 publications

(7 citation statements)

References 158 publications

(193 reference statements)

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“…The relative population of these states depends on the total column density and the excitation temperature (assuming a single, thermal population of absorbing molecular hydrogen [85]). Measuring numerous absorption features for 22 CTTSs, Hoadley et al [86] find that (a) the low-energy states follow the pattern expected for T ∼ 2000 K while the high-energy states of H 2 show higher occupancies than expected. These authors speculate that the non-thermal populations arise in a diffuse molecular disk atmosphere where level populations are driven out of thermal equilibrium by Lyα photon pumping.…”

Section: Fluorescently Excited Molecular Emission Linesmentioning

confidence: 89%

“…In particular, these authors compare the radial distributions of full and transitional disks ( Figure 10), finding that the distribution in transitional disks is shifted towards larger radii. The H 2 and CO gas populations decline with dust disk dissipation [86,115], but the H 2 depletion lags behind the CO for disks with large inner dust cavities [84].…”

Section: Disk Dispersalmentioning

confidence: 99%

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The UV Perspective of Low-Mass Star Formation

Schneider¹,

Günther²

2020

Galaxies

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The formation of low-mass (M 2 M ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited to study star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.CTTSs were initially identified as a new class of objects due their optical variability. Very early on, it was realized that they also show signs of enhanced chromospheric activity, i.e., "emission lines resembling those of the solar chromosphere" [2]. These emission lines are superposed on a photospheric spectrum similar to main sequence stars of late spectral type. Initially, the source of emission in excess of a normal photosphere was speculated to be circumstellar or chromospheric in origin [3].The discovery that CTTSs are above and to the right of the main sequence in an HR-diagram demonstrated in conjunction with theoretical work [4], that these objects must be young and, thus, the progenitors of the large main sequence population [5,6]. This notion is corroborated by the presence of strong Li absorption, which requires ∼ 100× the Li abundance of the Sun [e.g. 7]. Because Li is depleted quickly in stellar interiors, the amount of surface lithium decreases with time in convective stars. Therefore, strong Li absorption can only be found in young stars and thus CTTS must be young [e.g. review by 8].Meanwhile the number of peculiarities found in CTTSs grew, but their established youth alone was insufficient to explain those peculiarities and the "mysteries" of CTTSs remained. Many features, like the origin of the "chromospheric emission" remained unexplained, until the early 1980s when a large variety of models were proposed to explain features of CTTSs including envelopes, outflows, infall, and dust disks.Specifically, the "ultraviolet excess" (measured around 3700 Å) and the "blue continuum", which go along with a weakening of photosperic absorption lines due to veiling [9,10], were shown to correlate with the strength of Hα [11,12]. Such a correlation indicates a common origin of both features as observed for chromospheric emission on the Sun. Thus, it was natural to ascribe the excess continuum emission at short wavelengths to a Balmer continuum, i.e., emission following the capture of a free electron by an ionized hydrogen atom. While that finding pinpoints neither the physical origin nor location of the emitting material, it paved the way for explanations involving a suitably tuned chromosphere for producing the observed (Balmer & Ca II H+K) line fluxes in CTTSs. Quantitative mod...

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Section: Fluorescently Excited Molecular Emission Linesmentioning

confidence: 89%

Section: Disk Dispersalmentioning

confidence: 99%

The UV Perspective of Low-Mass Star Formation

Schneider¹,

Günther²

2020

Galaxies

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“…Both the G130M and G160M modes were used for most targets, providing wavelength coverage from ∼1100 to 1700 Å. Various features in the spectra have been presented in previous work, including accretion rate diagnostics (Ardila et al 2013), fluorescent emission lines from excited electronic states of hot H 2 (France et al 2012;Hoadley et al 2015), 12 CO and 13 CO A − X absorption bands (McJunkin et al 2013), Lyα emission (Schindhelm et al 2012b) and H 2 absorption along the Lyα profile (Hoadley et al 2017), the far-UV radiation field (Yang et al 2012;France et al 2014b), and connections to UV photochemistry (Arulanantham et al 2020).…”

Section: Targets and Observationsmentioning

confidence: 99%

UV Fluorescence Traces Gas and Lyα Evolution in Protoplanetary Disks

Arulanantham

France

Hoadley

et al. 2021

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Ultraviolet spectra of protoplanetary disks trace distributions of warm gas at radii where rocky planets form. We combine Hubble Space Telescope Cosmic Origins Spectrograph observations of H 2 and CO emission from 12 classical T Tauri stars to more extensively map inner disk surface layers, where gas temperature distributions allow radially stratified fluorescence from the two species. We calculate empirical emitting radii for each species under the assumption that the line widths are entirely set by Keplerian broadening, demonstrating that the CO fluorescence originates further from the stars ( ) r 20 au than the H 2 ( ) r 0.8 au . This is supported by 2D radiative transfer models, which show that the peak and outer radii of the CO flux distributions generally extend further into the outer disk than the H 2 . These results also indicate that additional sources of Lyα photons remain unaccounted for, requiring more complex models to fully reproduce the molecular gas emission. As a first step, we confirm that the morphologies of the UV-CO bands and Lyα radiation fields are significantly correlated and discover that both trace the degree of dust disk evolution. The UV tracers appear to follow the same sequence of disk evolution as forbidden line emission from jets and winds, as the observed Lyα profiles transition between dominant red wing and dominant blue wing shapes when the high-velocity optical emission disappears. Our results suggest a scenario where UV radiation fields, disk winds and jets, and molecular gas evolve in harmony with the dust disks throughout their lifetimes.

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“…Both the G130M and G160M modes were used for most targets, providing wavelength coverage from ∼1100-1700 Å. Various features in the spectra have been presented in previous work, including accretion rate diagnostics (Ardila et al 2013), fluorescent emission lines from excited electronic states of hot H 2 (France et al 2012;Hoadley et al 2015), 12 CO and 13 CO A − X absorption bands (McJunkin et al 2013), Lyα emission (Schindhelm et al 2012b) and H 2 absorption along the Lyα profile (Hoadley et al 2017), the FUV radiation field (Yang et al 2012;France et al 2014b), and connections to UV photochemistry (Arulanantham et al 2020).…”

Section: Targets and Observationsmentioning

confidence: 99%

UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks

Arulanantham,

France,

Hoadley

et al. 2021

Preprint

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Ultraviolet spectra of protoplanetary disks trace distributions of warm gas at radii where rocky planets form. We combine HST -COS observations of H 2 and CO emission from 12 classical T Tauri stars to more extensively map inner disk surface layers, where gas temperature distributions allow radially stratified fluorescence from the two species. We calculate empirical emitting radii for each species under the assumption that the line widths are entirely set by Keplerian broadening, demonstrating that the CO fluorescence originates further from the stars (r ∼ 20 AU) than the H 2 (r ∼ 0.8 AU). This is supported by 2-D radiative transfer models, which show that the peak and outer radii of the CO flux distributions generally extend further into the outer disk than the H 2 . These results also indicate that additional sources of Lyα photons remain unaccounted for, requiring more complex models to fully reproduce the molecular gas emission. As a first step, we confirm that the morphologies of the UV-CO bands and Lyα radiation fields are significantly correlated and discover that both trace the degree of dust disk evolution. The UV tracers appear to follow the same sequence of disk evolution as forbidden line emission from jets and winds, as the observed Lyα profiles transition between dominant red wing and dominant blue wing shapes when the high-velocity optical emission disappears. Our results suggest a scenario where UV radiation fields, disk winds and jets, and molecular gas evolve in harmony with the dust disks throughout their lifetimes.

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Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations

Cited by 8 publications

References 158 publications

The UV Perspective of Low-Mass Star Formation

The UV Perspective of Low-Mass Star Formation

UV Fluorescence Traces Gas and Lyα Evolution in Protoplanetary Disks

UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks

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