A global view on star formation: The GLOSTAR Galactic plane survey

Abstract

Context. Cygnus X is one of the closest and most active high-mass star-forming regions in our Galaxy, making it one of the best laboratories for studying massive star formation. Aims. We aim to investigate the properties of molecular gas structures on different linear scales with the 4.8 GHz formaldehyde (H2CO) absorption line in Cygnus X. Methods. As part of the GLOSTAR Galactic plane survey, we performed large-scale (7º×3º) simultaneous H2CO (11,0–11,1) spectral line and radio continuum imaging observations toward Cygnus X at λ ~6 cm with the Karl G. Jansky Very Large Array and the Effelsberg 100 m radio telescope. We used auxiliary HI, 13CO (1–0), dust continuum, and dust polarization data for our analysis. Results. Our Effelsberg observations reveal widespread H2CO (11,0–11,1) absorption with a spatial extent of ≳50 pc in Cygnus X for the first time. On large scales of 4.4 pc, the relative orientation between the local velocity gradient and the magnetic field tends to be more parallel at H2 column densities of ≳1.8×1022 cm−2. On the smaller scale of 0.17 pc, our VLA+Effelsberg combined data reveal H2CO (11,0–11,1) absorption only towards three bright HII regions. Our observations demonstrate that H2CO (11,0–11,1) is optically thin in general. The kinematic analysis supports the assertion that molecular clouds generally exhibit supersonic motions on scales of 0.17−4.4 pc. We show a non-negligible contribution of the cosmic microwave background radiation to the extended absorption features in Cygnus X. Our observations suggest that H2CO (11,0–11,1) can trace molecular gas with H2 column densities of ≳5 × 1021 cm−2 (i.e., AV ≳ 5). The ortho-H2CO fractional abundance with respect to H2 has a mean value of 7.0 × 10−10. A comparison of the velocity dispersions on different linear scales suggests that the velocity dispersions of the dominant −3 km s−1 velocity component in the prominent DR21 region are nearly identical on scales of 0.17−4.4 pc, which deviates from the expected behavior of classic turbulence.

Calculating the Excitation Temperature for H2CO Absorption Lines in Molecular Clouds

“…The column density of H 2 CO in the upper energy level, N u , may be estimated from the total H 2 CO column density N tot . For the H 2 CO (1 10 −1 11 ) absorption in the H II region of Cygnus X, Gong et al (2023) adopted an empirical relation between N u and N tot : N u /N tot ; 0.39 for a representative kinetic temperature of 10 K. This ratio may be different for different regions, but it should not vary too much under similar physical conditions. While there can be a 13% uncertainty in the derived column densities based on the above empirical relation, we have estimated the uncertainty in T ex caused by this 13% uncertainty using Monte Carlo simulations and found that the uncertainty in T ex is less than 0.1 K.…”

“…Of course, the ratio of column densities between H 2 CO and H 2 may not be equal everywhere, and the ratio of column densities between H 2 CO and 13 CO may also not be equal everywhere. These variations could introduce uncertainty in the H 2 CO column density, which may vary by about an order of magnitude (Gong et al 2023;Tang et al 2018). For some representative values of T L , T c and ΔV, we can describe how T ex and τ vary with changing N u , as shown in Figure 2.…”

“…In the absence of multiple transitions or hyperfine structures of a single transition, the distribution of the excitation temperature is difficult to be determined and a constant excitation temperature is usually assumed for the entire area, such as in Tang et al (2013), Komesh et al (2019) and Gong et al (2023). In a star formation region with varied environmental conditions, the assumption of a fixed value of T ex may result in under-or over-estimates of other physical parameters.…”

Magnetically Aligned Striations in the L914 Filamentary Cloud

“…The integral velocity interval is [−100, 40] km s −1 . Around the center of the Cygnus X region (the Cyg OB2 cluster), there are many bright filamentary molecular clouds, and a lot of research has focused on these molecular clouds over the past few decades (e.g., Schneider et al 2010;Cao et al 2022;Gong et al 2023;Li et al 2023). However, the relatively isolated L914 molecular cloud located at the periphery of this region has received little attention.…”