CONSTRAINING THE X-RAY AND COSMIC-RAY IONIZATION CHEMISTRY OF THE TW Hya PROTOPLANETARY DISK: EVIDENCE FOR A SUB-INTERSTELLAR COSMIC-RAY RATE

Journal Article · Sun Feb 01 04:00:00 EST 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/799/2/204· OSTI ID:22364290

Cleeves, L. Ilsedore; Bergin, Edwin A.; Adams, Fred C. ^[1]; Qi, Chunhua; Öberg, Karin I. ^[2]

---

1. Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109 (United States)
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

We present an observational and theoretical study of the primary ionizing agents (cosmic rays (CRs) and X-rays) in the TW Hya protoplanetary disk. We use a set of resolved and unresolved observations of molecular ions and other molecular species, encompassing 11 lines total, in concert with a grid of disk chemistry models. The molecular ion constraints comprise new data from the Submillimeter Array on HCO{sup +}, acquired at unprecedented spatial resolution, and data from the literature, including ALMA observations of N{sub 2}H{sup +}. We vary the model incident CR flux and stellar X-ray spectra and find that TW Hya's HCO{sup +} and N{sub 2}H{sup +} emission are best-fit by a moderately hard X-ray spectra, as would be expected during the ''flaring'' state of the star, and a low CR ionization rate, ζ{sub CR} ≲ 10{sup –19} s{sup –1}. This low CR rate is the first indication of the presence of CR exclusion by winds and/or magnetic fields in an actively accreting T Tauri disk system. With this new constraint, our best-fit ionization structure predicts a low turbulence ''dead-zone'' extending from the inner edge of the disk out to 50-65 AU. This region coincides with an observed concentration of millimeter grains, and we propose that the inner region of TW Hya is a dust (and possibly planet) growth factory as predicted by previous theoretical work.

• OSTI ID: 22364290
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 799; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCRETION DISKS
CONCENTRATION RATIO
COSMIC DUST
COSMIC RADIATION
HARD X RADIATION
IONIZATION
LIMITING VALUES
MAGNETIC FIELDS
MOLECULAR IONS
PLANETS
PROTOPLANETS
SPATIAL RESOLUTION
STARS
STELLAR WINDS
TURBULENCE
X-RAY SPECTRA