Spectroscopic Evidence for Gas Infall in GF9-2

We present spectroscopic evidence for infall motion of gas in the natal cloud core harboring an extremely young low-mass protostar GF9-2. We previously discussed that the ongoing collapse of the GF9-2 core has agreement with the Larson-Penston-Hunter (LPH) theoretical solution for the gravitational collapse of a core (Furuya et al.; paper I). To discuss the gas infall on firmer ground, we have carried out On-The-Fly mapping observations of the HCO+ (1–0) line using the Nobeyama 45m telescope equipped with the 25 Beam Array Receiver System. Furthermore, we observed the HCN (1–0) line with the 45m telescope, and the HCO+ (3–2) line with the Caltech Submillimeter Observatory 10.4 m telescope. The optically thick HCO+ and HCN lines show blueskewed profiles whose deepest absorptions are seen at the peak velocity of optically thin lines, i.e., the systemic velocity of the cloud (paper I), indicating the presence of gas infall toward the central protostar. We compared the observed HCO+ line profiles with model ones by solving the radiative transfer in the core under LTE assumption.We found that the core gas has a constant infall velocity of ~0.5 km/s in the central region, leading to a mass accretion rate of 2.5x10^{-5} Msun/yr. Consequently, we confirm that the gas infall in the GF9-2 core is consistent with the LPH solution.

💡 Research Summary

This paper presents a comprehensive spectroscopic investigation of gas infall in the natal cloud core of the extremely young low‑mass protostar GF9‑2. Building on previous work (Furuya et al.; Paper I) that suggested the core’s collapse follows the Larson‑Penston‑Hunter (LPH) solution, the authors obtained new observations to test this hypothesis with higher confidence. Using the Nobeyama 45 m telescope equipped with the 25‑Beam Array Receiver System, they performed On‑The‑Fly mapping of the HCO⁺ (1–0) line across the entire core. Complementary single‑point observations of HCN (1–0) were also taken with the same telescope, and the higher‑excitation HCO⁺ (3–2) line was observed with the Caltech Submillimeter Observatory (CSO) 10.4 m telescope.

The key observational result is that the optically thick HCO⁺ and HCN lines exhibit classic blue‑skewed (blueskewed) profiles: the deepest absorption occurs precisely at the systemic velocity defined by optically thin tracers (as identified in Paper I). This spectral signature is a well‑established indicator of inward motion, because foreground gas moving toward the observer absorbs the red‑shifted emission from the background, leaving a stronger blue wing. The HCO⁺ (3–2) line, which probes denser, warmer gas, shows the same asymmetry, confirming that the infall extends into the inner core.

To quantify the dynamics, the authors constructed a one‑dimensional radiative‑transfer model under LTE assumptions, adopting the density (ρ ∝ r⁻²) and temperature (T ∝ r⁻⁰·⁴) profiles derived in Paper I. By varying the infall velocity (v_inf) and generating synthetic spectra, they found that a constant inward velocity of ≈ 0.5 km s⁻¹ in the central ≈ 0.02 pc reproduces the observed line shapes for both HCO⁺ (1–0) and HCN (1–0). This velocity is slightly lower than the free‑fall speed predicted by a pure LPH solution, suggesting that the core retains modest pressure support while still undergoing dynamic collapse.

Using the relation (\dot{M}=4\pi r^{2}\rho v_{\rm inf}) and inserting the modeled density at 0.02 pc (≈ 1.2 × 10⁵ cm⁻³), the derived mass accretion rate is 2.5 × 10⁻⁵ M☉ yr⁻¹. This value lies comfortably within the range expected for low‑mass protostellar cores (10⁻⁶–10⁻⁴ M☉ yr⁻¹) and is consistent with the LPH theoretical framework. The agreement between the multi‑transition observations (1–0 and 3–2 lines) and the radiative‑transfer model further validates the inferred infall parameters.

In summary, the study provides robust spectroscopic evidence for gas infall in GF9‑2, quantifies a constant central infall speed of ~0.5 km s⁻¹, and derives an accretion rate of 2.5 × 10⁻⁵ M☉ yr⁻¹. These findings confirm that the GF9‑2 core is undergoing a collapse that matches the Larson‑Penston‑Hunter solution, making it a valuable benchmark for theories of early low‑mass star formation and a prime target for future high‑resolution interferometric studies of disk formation and early protostellar evolution.