Pre-main-sequence Brackett Emitters in the APOGEE DR17 Catalog: Line Strengths and Physical Properties of Accretion Columns

Abstract

Very young (t ≲ 10 Myr) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar disks to the surface of the star. Upon impacting the stellar surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11–20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main-sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of n e = 1011–1012 cm−3, and excitation temperatures that are inversely correlated with electron density (from T ∼ 5000 K for n e ∼ 1012 cm−3 to T ∼ 12,500 K at n e ∼ 1011 cm−3). These physical parameters are in good agreement with predictions from modeling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission-line sources in the APOGEE Data Release 17 data set.

ABYSS. I. Targeting Strategy for the APOGEE and BOSS Young Star Survey in SDSS-V

“…The previous iterations of SDSS have produced a spectroscopic census of young stars across several selected starforming regions. These data have been instrumental in understanding the star formation history and the threedimensional kinematics of star-forming regions as a whole (e.g., Foster et al 2015;Da Rio et al 2016;Stutz & Gould 2016;Galli et al 2019;Kounkel et al 2022c), as well as properties of individual stars, such as accretion (Campbell et al 2023), multiplicity (Kounkel et al 2019), evolutionary properties (e.g., Serna et al 2021;Cao et al 2022), and stellar parameters (e.g., Roman-Lopes et al 2019;Ramírez-Preciado et al 2020). Similarly to SDSS, other spectroscopic surveys have targeted nearby star-forming regions, such as GALAH (Kos et al 2021), or with Gaia-ESO (e.g., Sacco et al 2015;Bouvier et al 2016).…”

ABYSS. II. Identification of Young Stars in Optical SDSS Spectra and Their Properties

“…There is no strong correlation with the properties of the star itself, such as T eff , g log , age, or any other. For comparison, Campbell et al (2023) find a much larger range in Temp, from 4000 to 16,000 K (with the sources with hotter Temp preferentially being Be stars) and density ranging from ∼10 12 cm −3 at 5000 K, to ∼10 11 cm −3 at 12,500 K. Over the range of the overlapping Temp, there is a relative agreement in the derived log n.…”

“…Using a custom-built interactive code previously used in Campbell et al (2023), we recorded, for each line in each source, the wavelength and flux (λ 1 , F 1 ) near the start of the line as well as those near the end of the line (λ 2 , F 2 ). These set of points define the continuum under which the spectrum was integrated to estimate EqW (Figure 1).…”

“…Of 6444 CTTSs, 3085 show emission in Hβ, 1966in Hγ, 1215in Hδ, 1021 When multiple lines are available, because each of them has a different excitation temperature, it is possible to probe the physical conditions such as temperature (Temp) and density (log n) of the emitting gas (i.e., the accretion stream). This was previously done in Campbell et al (2023) using Brackett lines observed with APOGEE, through comparing the ratios of equivalent widths of these lines to the models from Kwan & Fischer (2011).…”

“…The strongest of those lines in the optical spectra is Hα, often presenting equivalent widths (EqW) of tens to hundreds of Å (White & Basri 2003). Particularly energetic shocks are capable of exciting other transitions of H (Kwan & Fischer 2011;Wilson et al 2022;Campbell et al 2023), as well as a number of other elements, including various O, Fe, He, S, N, and others (Hamann & Persson 1992a, 1992b, 1992cHamann 1994;Baldovin-Saavedra et al 2012;Ballabio et al 2023). Young stars exhibiting accretion are known as Classical T Tauri Stars (CTTSs).…”