(C) NASA
This story was originally published by NASA and is unaltered.
. . . . . . . . . .



SOFIA Observations of 30 Doradus. II. Magnetic Fields and Large-scale Gas Kinematics [1]

['Le Ngoc Tram', 'Max-Planck-Institut Für Radioastronomie', 'Auf Dem Hügel', 'Bonn', 'Germany', 'Nle Mpifr-Bonn.Mpg.De', 'Http', 'Lars Bonne', 'Stratospheric Observatory For Infrared Astronomy', 'Universities Space Research Association']

Date: 2024-07

In modern Astrophysics, magnetic fields (B-fields) and turbulence are believed to affect the star formation process. The B-fields support against gravitational collapse, while turbulence plays a dual role. Turbulence can against global cloud collapse, but can also produce local compression (Mac Low & Klessen 2004) with compressible and solenoidal motions acting in opposite directions (Cho & Lazarian 2003). The role of B-fields can be different depending on whether B-fields are dynamically important or subdominant (see a review in Crutcher 2012). For weak B-fields, the cloud is supercritical (the mass-to-flux ratio is greater than unity), and the B-fields are insufficient to prevent gravitational collapse. For strong B-fields, the fields are strong enough to counteract the collapse. In this case, other mechanisms must be invoked for star formation to occur in the subcritical cloud (the mass-to-flux ratio is lower than unity). There are two candidates for such mechanisms: (1) ambipolar diffusion (e.g., Mestel 1966) can increase the mass faster than the B-field strength, enhancing the gravitational counterpart, and (2) fast turbulent reconnection (Lazarian & Vishniac 1999) removes the magnetic flux, weakening the magnetic support. These two mechanisms are able to increase the mass-to-flux ratio, which can lead clouds to collapse and possibly to coexist (Lazarian 2014).

The fast turbulent reconnection induces a turbulence cascade perpendicular to the ambient B-fields. As a result, turbulent eddies can freely mix the B-fields parallel to the rotation axes, where the velocity gradients (VGs) are perpendicular to the local direction of B-fields. This is the basis of the velocity gradient technique (VGT; González-Casanova & Lazarian 2017) to study B-fields in diffuse gas (Yuen & Lazarian 2017a; Hu et al. 2018, 2019a; Lazarian & Yuen 2018; Lazarian et al. 2018), in molecular clouds (Hu et al. 2019b, 2021; Hu et al. 2022; Tang et al. 2019; Alina et al. 2022), and in the atomic–molecular transition (Skalidis et al. 2022). Nevertheless, self-gravity is able to break that relationship. The gravitational forces pull the gas in the direction along the B-fields, so the VGs are dominated by the infall acceleration. In this case, the VGs are parallel to the ambient B-fields. The misalignment between VGT and B-fields becomes a proxy for the gravitational collapse (Yuen & Lazarian 2017b; Lazarian & Yuen 2018; Tang et al. 2019; Hu et al. 2021). Therefore, VGT is a promising tool to probe the B-field morphology and the local gravitational collapse.

The B-fields, turbulence, and stellar feedback shape the cloud and regulate the star formation processes. The simulations with uniform B-fields from Henney et al. (2009), Mackey & Lim (2011) showed that the B-fields have a significant contribution in shaping the cloud. This result depends on the orientation between the initial B-fields and the radiation from the source. Specifically, these authors found that clouds are flatter (broad head) if the B-fields are parallel to the radiation direction, while the cloud becomes a more elongated structure (tail-like structure) if the B-fields are perpendicular to the radiation direction. These features appear to be confirmed from observations, e.g., IC 1396 (Soam et al. 2018a), M16 (Pattle et al. 2018), Ophiuchus-A (Santos et al. 2019). The simulations of the feedback in a turbulent magnetized cloud by Arthur et al. (2011) showed that the B-fields tend to be amplified and slow down the formation of stars.

The morphology of the B-fields is affected by gravity (resulting in a well-known hour-glass shape; Ewertowski & Basu 2013), and regulated by the supersonic gas motion as proposed by Inoue & Fukui (2013). The latter seems to be frequently observed, e.g., deformation of B-field geometry in M16 (Pattle et al. 2018), Orion-A (Tahani et al. 2019), Musca filament (Bonne et al. 2020a, 2020b), and BHR 71 bipolar outflow system (Kandori et al. 2020). Using simulations, Abe et al. (2021) demonstrated that a shock with a velocity of ∼7 km s−1 is able to wrap the B-fields.

Even though B-fields may affect the star formation processes, the direct measurement of B-fields is difficult. Alternatively, the B-fields are inferred using several data analysis methods. One of the methods consists in using dust polarization (see, e.g., Lazarian 2007; and Andersson et al. 2015 for reviews). The basic idea of this technique relies on the fact that irregular dust grains tend to align with their shortest axis parallel to the local B-fields due to various physical effects (see, e.g., Hoang et al. 2022a for details) so that their thermal emission is polarized with the polarization orientation perpendicular to the B-fields (see Tram & Hoang 2022). The measured position angle of thermal dust polarization is then perpendicular to the local B-fields in the plane of the sky. Hence, the polarimetric data allows us to map the B-field geometry by rotating the polarization angles by 90o . The polarized thermal dust emission is feasible at long wavelengths, i.e., far-infrared (FIR) to submillimeter. The strength of the B-fields on the plane of the sky (B POS ) can be estimated using the Davis–Chandrasekhar–Fermi (DCF; Davis 1951; Chandrasekhar & Fermi 1953) method. This method is commonly used, although some modifications need to be taken into account as a function of the object to be analyzed (Liu et al. 2022). Another approach (namely the differential measure approach, or DMA) is recently proposed by Lazarian et al. (2020, 2022), which is suggested to be able to measure the B-field strength more precisely.

Our target is the star formation region 30 Doradus (hereafter 30 Dor) in the Large Magellanic Cloud (LMC). With a distance of ≃50 kpc away from Earth (Schaefer 2008), it is close enough to obtain parsec-scale resolutions to study the impact of the feedback and turbulence on the surrounding molecular cloud. 30 Dor hosts a massive star cluster, R136, which is associated to the H ii giant expanding-shells (Kennicutt 1984; Chu & Kennicutt 1994; Brandl 2005; Townsley et al. 2006; Lopez et al. 2011), and a nearby supernova remnant (Townsley et al. 2006). Figure 1 16 shows a composite image of the 30 Dor region with an overlay of the field of view covered in our study. This complex system is embedded by multiple H i giant-shells (Kim et al. 1999). A combination of stellar winds and supernovae (Chu & Kennicutt 1994) or only the cluster-wind (not the stellar wind of individual stars) from R136 (Melnick et al. 2021) are demonstrated to be the main sources to create these giant H ii expanding-structures. The authors also clearly unveiled two structures in 30 Dor. For the nebula's core (within a distance of 25 pc proximity to R136), surprisingly, the thermal gas pressure is lower than that of the stellar radiation (see Figure 18 in Pellegrini et al. 2011), and the mass is lower than the virial mass (Melnick et al. 2021). Hence, the important questions remaining are as follows: How can this structure survive? And how can stars form? (The locations of protostar candidates are shown in, e.g., Lee et al. 2019; Indebetouw et al. 2009). Here, we focus on the closest region to R136, which is indicated by the white box in Figure 1. For the sake of simplicity, we refer to this region as 30 Dor in this work.

Figure 1. Public composite image of 30 Dor observed by La Silla 2.2 m telescope with H α -658.827 nm (red), a combination of V-539.562 nm and [O iii]-502.393 nm (green), and B-451.100 nm (blue). This image shows a complex structure of the region with multiple large expanding-shells produced by the hot cluster-wind from R136 (indicated by a red star), and a slow expanding-shell from the supernova remnant 30DorB (lower right). The white box shows the region covered by SOFIA/HAWC+ that we analyze in this work. Download figure: Standard image High-resolution image

Our goals are to

1. map the morphology and strength of the B-fields in 30 Dor using FIR polarimetric observations with SOFIA/HAWC+ as introduced in Tram et al. ( 2021c ; hereafter Paper I );

2. ii ] and CO(2-1) data acquired by SOFIA/GREAT and APEX (see Okada et al. examine the gas kinematic in 30 Dor by making use of [C] and CO(2-1) data acquired by SOFIA/GREAT and APEX (see Okada et al. 2019 );

3. make use of the VGT to probe the local gravitational collapse in 30 Dor;

4. quantify the effect of B-fields on supporting the cloud integrity; and

5. perform an energy budget to quantify the effect of gravity, B-fields, and turbulence on the star-forming processes of 30 Dor.

This paper is structured as follows. We analyze the gas kinematics in Section 2. The analysis of the B-field orientations and strengths are shown in Section 3. Our discussions are presented in Section 4. The conclusions are presented in Section 5.

[END]
---
[1] Url: https://iopscience.iop.org/article/10.3847/1538-4357/acaab0

Published and (C) by NASA
Content appears here under this condition or license: Public Domain.

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/nasa/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.