On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga

By Russel J. White, L. Hillenbrand. 2004

289 citations recorded by the source corpus.

We present high-resolution (R ~ 34,000) optical (6330-8750 Å) spectra obtained with the HIRES spectrograph on the W. M. Keck I telescope of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. The sample includes 15 class I stars and all class II stars that power Herbig-Haro flows in this region. For 28 of the 36 stars observed, our measurements are the first high-dispersion optical spectra ever obtained. Photospheric features are observed in all stars with detected continuum, 11 of 15 class I stars (42% of known Taurus class I stars) and 21 of 21 class II stars; strong emission lines (e.g., Hα) are detected in the spectra of all stars. These spectra, in combination with previous measurements, are used to search for differences between stars that power Herbig-Haro flows and stars that do not and to reassess the evolutionary state of so-called protostars (class I stars) relative to optically revealed T Tauri stars (class II stars). The stellar mass distribution of class I stars is similar to that of class II stars and includes three spectroscopically confirmed class I brown dwarfs. Class I stars (and brown dwarfs) in Taurus are slowly rotating (v sin i < 35 km s-1); the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The amount of optical veiling and the inferred mass accretion rates of class I stars are surprisingly indistinguishable from class II stars. Class I stars do not have accretion-dominated luminosities; the accretion luminosity accounts for ~25% of the bolometric luminosity. The median mass accretion rate of class I and class II stars of K7-M1 spectral type is 4 × 10-8 M☉ yr-1, and the median mass outflow rate is 5% of the mass accretion rate. The large ranges in mass accretion rate (~2 orders of magnitude), mass outflow rate (~3 orders of magnitude), and the ratio of these quantities (~2 orders of magnitude) represent real dispersions in young accreting stars of similar mass. We confirm previous results that find larger forbidden-line emission associated with class I stars than class II stars. We suggest that this is caused by an orientation bias that allows a more direct view of the somewhat extended forbidden emission line regions than of the obscured stellar photospheres, rather than being caused by larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of class I stars and class II stars are best explained by a scenario in which most class I stars are no longer in the main accretion phase and are much older than traditionally assumed. Similarly, although stars that power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of similar mass stars that do not power these flows.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/408d4572fd236f6c0188fe98ff985ffc84878113

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Citations in the corpus

AN OPTICAL SPECTROSCOPIC STUDY OF T TAURI STARS. I. PHOTOSPHERIC PROPERTIES (2014, methodology): “In some cases, high resolution spectra can yield enough photospheric absorption lines to reveal a spectral type (White & Hillenbrand 2004).”
Accretion onto Pre-Main-Sequence Stars (2016, background): “The most sensitive optical survey of protostars is that of White & Hillenbrand (2004), who studied 15 Class I sources in Taurus in the red-optical region.”
A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus (2006, background): “The flat-spectrum sources in this figure are objects associated with Herbig-Haro jets and nebulosities in their surroundings (White & Hillenbrand 2004; Krist et al. 1998 and references therein).”
Demographics of Transition Objects (2007, unknown): “A demographic approach has the advantage of being robust against systematic errors, since systematic errors (e.g., of the kind described by White & Hillenbrand 2004 and Hartmann et al. 2006) can shift demographic trends up or down in Ṁ∗ or Mdisc but should not obscure them.”
FIRST RESULTS FROM HIGH ANGULAR RESOLUTION ALMA OBSERVATIO NS TOWARD THE HL TAU REGION (2015, background): “Based on high dispersion optical spectroscopy, HL Tau is classified as spectral type K5±1 (White & Hillenbrand 2004).”
The Sun in Time: Activity and Environment (2007, background): “Somewhat perplexingly, the analysis of the Taurus sample of embedded objects reveals very little statistical difference between Class I and CTTS objects in Taurus, for example with respect to rotation rates, accretion rates, bolometric luminosities, spectral classes, and disk masses, providing evide”
Theory of Star Formation (2007, background): “Recent observational studies of the central stars in Class I sources differ on their evolutionary status: White & Hillenbrand (2004) argue that the Class I protostars are similar to T Tauri stars and are thus past the main protostellar accretion phase, whereas Doppmann et al. (2005) come to the…”
UV Excess Measures of Accretion onto Young Very Low Mass Stars and Brown Dwarfs (2008, background): “White & Hillenbrand (2004) suggest that evolutionary tracks underestimate stellar masses by 30– 50%.”
ACCRETION RATES FOR T TAURI STARS USING NEARLY SIMULTANEOUS ULTRAVIOLET AND OPTICAL SPECTRA (2013, background): “Veiling at progressively redder wavelengths can be found in the literature, out to 1 µm (Basri & Batalha 1990; Hartigan et al. 1991; White & Hillenbrand 2004; Edwards et al. 2006).”
Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective (2005, background): “…1986; Herbig & Bell 1988; Strom et al. 1989; Hartmann et al. 1991; Gomez et al. 1992; Bouvier et al. 1993; Briceño et al. 1993; Hartigan, Strom, & Strom 1994; Mart́ın et al. 1994; Kenyon & Hartmann 1995; Hernández et al. 2004; White & Hillenbrand 2004, spectral types are listed in Table 1).”
Probing T Tauri Accretion and Outflow with 1 Micron Spectroscopy (2006, background): “However, distributions of veiling among similar ensembles of CTTS indicate that it appears to be relatively constant from 0.5 µm to 0.8 µm (Basri & Batalha 1990; White and Hillenbrand 2004), suggesting that there is – 23 – an additional contribution to emission excess.”
PHOTOEVAPORATION OF CIRCUMSTELLAR DISKS BY FAR-ULTRAVIOLET, EXTREME-ULTRAVIOLET AND X-RAY RADIATION FROM THE CENTRAL STAR (2008, background): “Accretion is also accompanied by protostellar winds originating from the inner disk that are proportional to the rate of accretion (Ṁw ∼ 0.05–0.1Ṁacc; White & Hillenbrand 2004).”