Effect of L-Histidine on the Survival of a T-Strain of Mycoplasma

Ajello, Francesca; Romano, Nino

doi:10.1128/aem.29.2.293-294.1975

Cited by 5 publications

(8 citation statements)

References 1 publication

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“…Without this wavelength region, spectral fits to the data will not account for the Werner lines, biasing the results toward very low energy electron distributions. As the threshold energies for the Lyman and Werner bands are 11.37 and 12.41 eV (Ajello et al 1984), respectively, Lyman band continuum emission in the absence of discrete Werner band lines requires a highly-tuned electron distribution. Because Lyα production from the dissociation of H 2 drops sharply at 14.7eV (Ajello et al 1991), discrete lines should be even more important relative to the continuum at very low electron energies.…”

Section: Outstanding Issues Regarding Electron-impactmentioning

confidence: 99%

“…This mechanism requires stellar X-rays to create a distribution of photoelectrons that pump the molecules into excited electronic states. The H 2 then relaxes, producing a characteristic far-UV cascade spectrum of discrete emission lines and quasi-continuous spectral features during the dissociation of H 2 (Ajello et al 1984;Abgrall et al 1997). Ingleby et al (2009) proposed this mechanism to explain a portion of the low-resolution far-UV spectra from a sample of classical and weak-lined (low gas content) T Tauri stars observed with the Hubble Space Telescope-ACS and -STIS.…”

Section: Introductionmentioning

confidence: 99%

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THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

Yang

Linsky

2011

ApJ

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We present deep spectroscopic observations of the classical T Tauri stars DF Tau and V4046 Sgr in order to better characterize two important sources of far-ultraviolet continuum emission in protoplanetary disks. These new Hubble Space Telescope-Cosmic Origins Spectrograph observations reveal a combination of line and continuum emission from collisionally excited H 2 and emission from accretion shocks. H 2 is the dominant emission in the 1400 λ 1650Å band spectrum of V4046 Sgr, while an accretion continuum contributes strongly across the far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046 Sgr to models of electron-impact induced H 2 emission to constrain the physical properties of the emitting region, after making corrections for attenuation within the disk. We find reasonable agreement with the broad spectral characteristics of the H 2 model, implying N (H 2 ) ∼ 10 18 cm −2 , T (H 2 ) = 3000 +1000 −500 K, and a characteristic electron energy in the range of ∼ 50 -100 eV. We propose that self-absorption and hydrocarbons provide the dominant attenuation for H 2 line photons originating within the disk. For both DF Tau and V4046 Sgr, we find that a linear fit to the far-UV data can reproduce near-UV/optical accretion spectra. We discuss outstanding issues concerning how these processes operate in protostellar/protoplanetary disks, including the effective temperature and absolute strength of the radiation field in low-mass protoplanetary environments. We find that the 912 -2000Å continuum in low-mass systems has an effective temperature of ∼ 10 4 K with fluxes 10 5−7 times the interstellar level at 1 AU.

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Section: Outstanding Issues Regarding Electron-impactmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

Yang

Linsky

2011

ApJ

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“…In the general form, Equation A1 must also be summed over the possible redistribution over rotational (∆J = ±1, 0) states, however, the present case is simplified due to the parity selection rules that forbid Q branch (∆J = 0) transitions to mix with R (∆J = -1) and P (∆J = +1) branches. The predissociation fraction for the Werner bands is zero (ξ C = 0; Ajello et al, 1984).…”

Section: O VI Flux Limitsmentioning

confidence: 99%

METAL DEPLETION AND WARM H₂IN THE BROWN DWARF 2M1207 ACCRETION DISK

Linsky

Brown

Froning

et al. 2010

ApJ

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We present new far-ultraviolet observations of the young M8 brown dwarf 2MASS J12073346-3932539, which is surrounded by an accretion disk.The data were obtained using the Hubble Space T elescope-Cosmic Origins Spectrograph. Moderate resolution spectra (R ≈ 17,000 -18,000) obtained in the 1150 -1750 Å and 2770 -2830 Å bandpasses reveal H 2 emission excited by H I Lyα photons, several ionization states of carbon (C I -C IV), and hot gas emission lines of He II and N V (T ≈ 10 4 -10 5 K). Emission from some species that would be found in a typical thermal plasma at this temperature (Si II, Si III, Si IV, and Mg II) are not detected. The non-detections indicate that these refractory elements are depleted into grains, and that accretion shocks dominate the production of the hot gas observed on 2MASS J12073346-3932539. We use the observed C IV luminosity to constrain the mass accretion rate in this system. We use the kinematically broadened H 2 profile to confirm that the majority of the molecular emission arises in the disk, measure the radius of the inner hole of the disk (R hole ≈ 3R * ), and constrain the physical conditions of the warm molecular phase of the disk (T (H 2 ) ≈ 2500 -4000 K). A second, most likely unresolved H 2 component is identified. This feature is either near the stellar surface in the region of the accretion shock or in a molecular outflow, although the possibility that this Jovian-like emission arises on the day-side disk of a 6 M J companion (2M1207b) cannot be conclusively ruled out. In general, we find that this young brown dwarf disk system is a low-mass analog to classical T Tauri stars that are observed to produce H 2 emission from a warm layer in their disks, such as the well studied TW Hya and DF Tau systems.

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“…In the case of AU Mic, we are concerned with Werner band emission (n ′ -n ′′ = C -X ≡ C 1 Π u -X 1 Σ + g ), v ′ = 1, and v ′′ = 3 and 4 for the 1119.08 and 1163.81Å lines, respectively. The predissociation fraction for the Werner bands is zero (ξ C = 0; Ajello et al, 1984).…”

Section: Total Emitted H 2 Fluxmentioning

confidence: 99%

A Low‐Mass H₂Component to the AU Microscopii Circumstellar Disk

Roberge

Lupu

Redfield

et al. 2007

ApJ

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We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H 2 , pumped by the O VI λ1032 resonance line through the C-X (1 -1) Q(3) λ1031.87Å transition, are detected by the F ar U ltraviolet Spectroscopic Explorer. These lines are used to derive the H 2 column density associated with the AU Mic system. The derived column density is in the range N (H 2 ) = 1.9 × 10 17 -2.8 × 10 15 cm −2 , roughly two orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H 2 excitation temperature consistent with the observations, T (H 2 ) = 800 -2000 K, derived from the presence of emission lines excited by O VI in the absence of those excited by Lyα. Within the observational uncertainties, the data are consistent with the H 2 gas residing in the disk. The inferred N (H 2 ) range corresponds to H 2 -to-dust ratios of 1 30 :1 and a total M (H 2 ) = 4.0 × 10 −4 -5.8 × 10 −6 M ⊕ . We use these results to predict the intensity of the associated rovibrational emission lines of H 2 at infrared wavelengths covered by ground-based instruments, HST -NICMOS, and the Spitzer-IRS. Subject headings: circumstellar matter -stars: individual (HD 197481, AU Microscopii) -planetary systems: protoplanetary disks -ultraviolet: stars 1 Based in part on observations made with the the F ar U ltraviolet Spectroscopic Explorer, operated by the Johns Hopkins University for NASA.

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Effect of L-Histidine on the Survival of a T-Strain of Mycoplasma

Cited by 5 publications

References 1 publication

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

METAL DEPLETION AND WARM H₂IN THE BROWN DWARF 2M1207 ACCRETION DISK

A Low‐Mass H₂Component to the AU Microscopii Circumstellar Disk

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Effect of L-Histidine on the Survival of a T-Strain of Mycoplasma

Cited by 5 publications

References 1 publication

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H2AND ACCRETION SHOCKS

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H2AND ACCRETION SHOCKS

METAL DEPLETION AND WARM H2IN THE BROWN DWARF 2M1207 ACCRETION DISK

A Low‐Mass H2Component to the AU Microscopii Circumstellar Disk

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THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

THE FAR-ULTRAVIOLET “CONTINUUM” IN PROTOPLANETARY DISK SYSTEMS. I. ELECTRON-IMPACT H₂AND ACCRETION SHOCKS

METAL DEPLETION AND WARM H₂IN THE BROWN DWARF 2M1207 ACCRETION DISK

A Low‐Mass H₂Component to the AU Microscopii Circumstellar Disk