The role of magnetic fields during massive star formation

Transcription

1 The role of magnetic fields during massive star formation Wouter Vlemmings Chalmers University of Technology / Onsala Space Observatory / Nordic ALMA Regional Center with: Gabriele Surcis, Kalle Torstensson, Richard Dodson, Ramiro Franco, Rosy Torres, Huib Jan van Langevelde, Sharmila Goedhart, Chema Torrelles and others.

2 Star formation magnetic fields Strongly magnetized ISM Weakly magnetized stars Strong increase in mass-to-flux ratio a classic problem in star formation (e.g. Mestel & Spitzer 1956) Magnetic fields essential in many current theories of jet launching angular momentum transfer Regulating molecular cloud core collapse Observations are few! 2 /21

3 State of the Art: Massive Star Formation Massive stars Inject significant amounts of energy into the ISM Produce all the heavy elements Low-mass star formation strongly influenced by massive stars Radiation pressure constraint General spherical considerations rule out M > 11M sun non-spherical accretion (disks?) Formation scenario yet unknown Competing formation scenarios: Core collapse and Competitive accretion Key Question: How does the magnetic field affect MSF? 3 /21

4 Competing Massive Star Formation Scenarios Modified low-mass star formation (core accretion): - Increased accretion rates - Disk geometry helps accretion processes - Radiation pressure escapes through outflow cavities Coalescence and competitive accretion: - Massive stars form only in clusters. - Fragmentation and accretion toward objects in the cluster center - At high densities protostars entities merge Bate, Bonnell et al. Pudritz & Banerjee 5 4 /21

5 Competing Massive Star Formation Scenarios Modified low-mass star formation (core accretion): - Increased accretion rates - Disk geometry helps accretion processes - Radiation pressure escapes through outflow cavities Coalescence and competitive accretion: - Massive stars form only in clusters. - Fragmentation and accretion toward objects in the cluster center - At high densities protostars entities merge How to differentiate between both scenarios? - Molecular outflows and accretion disks. - Fragmentation and global collapse. Bate, Bonnell et al. Pudritz & Banerjee 5 5 /21

6 Magnetic fields during massive SF Turbulence, feedback & magnetic fields? Outflows ubiquitous MHD launching? B-fields suppress fragmentation? Field (de)stabilize accretion disks? Most effects close to protostars Need high-density, small scale B-field tracer masers! Less fragmentation for increasing B- field (Bate et al. 7) 6 /21

7 Recent models D. Seifried et al. log(n [cm-2]) 26. t = yr log(n [cm-2]) 26. t = yr x / AU log(bz [G]) 25. y / AU y / AU t = yr y / AU x / AU 1 km/s x / AU 1 km/s 6 log(n [cm-2]) log(n [cm-2]) t = yr -4 - t = yr 1 km/s t = yr log(bz [G]) y / AU y / AU y / AU x / AU 6 1 km/s - - x / AU 6 1 km/s x / AU 6 1 km/s Only weakly magnetized cores form Keplerian disks if turbulence is not present Figure 5. Column density of the disc after yr (left) and yr (middle) for run 26-4 (top) and run (bottom). White dots mark the projected positions of the sink particles, black vectors the velocity field in the midplane. In run 26-4 the disc is well defined with its inner region being subject to fragmentation. In contrast, in run a disc with sub-keplerian motions forms. The bubble like structures in this run are caused by the high magnetic pressure in the midplane driving material outwards (see also Section 4.1). Right: z-component of the magnetic field in the midplane for run 26-4 (top) and run (bottom) at the end of the simulation, i.e. after yr. The tight correlation between field strength and matter is due to the condition of ideal MHD. Note the di erent spatial scales in the upper and lower panel. More strongly magnetized cores form sub-keplerian disks (unless turbulent motions relieve magnetic braking), less collimated outflows but with high accretion and little fragmentation Seifried et al. 211,213 7 /21

8 Polarized SF Masers Maser polarization observed from: OH (1.6 and 6 GHz) Faraday rotation H2O (22 GHz) Shocks SiO (43 and 86 GHz) rare, polarization interpretation CH3OH (methanol, 6.7, 12.2, 36 GHz) Common MSF maser, strong 8 /21

9 Methanol maser magnetic field observations 9 /21

10 The Zeeman splitting of 6.7 GHz methanol R-L splitting observed in 8 ~.1-.5% circular attributed to Zeeman splitting with a coefficient of 49 m/s Gauss -1 B-fields of ~13 mg But: Zeeman coefficient 5 m/s Gauss -1 B-fields order of magnitude higher unrealistic? non-zeeman or wrong coefficient? no indication of non-zeeman! [Anisotropic scattering??] ~1 mg on co-located 6 GHz OH masers Vlemmings et al. 8, 211 A&A New lab data needed to settle the issue! 1/21

11 Sample Results Significant detection in 33/44 SF regions <VZ>=.6 m/s ~13 mg? Field variation across maser spectrum typically within factor 3 Field reversal in 2 sources 11/21

12 Field mapping: Cepheus A HW 2 Cepheus A HW2 ~2 7 pc (Jiménez-Serra et al. 7; Moscadelli et al. 9) Thermal radio jet, ionized gas at ~5 km/s (Curiel et al. 6) Rotating dust (R~33 AU) and molecular gas (R~58 AU) disk structure to outflow (Patel et al. 5, Jiménez-Serra et al. 7, Torrelles et al. 7) made up of at least 3 YSOs (e.g. Comito et al. 7) Flattened 6.7 GHz methanol maser structure near disk plane (R~65 AU, h~3 AU) infall at ~1.7 km/s (Torstensson et al.) SO 2 12 /21

13 Field mapping: Cepheus A HW 2 MERLIN image of the polarization of methanol masers around the outflow Cepheus A HW2 75 AU 13/21

14 Field mapping: Cepheus A HW 2 MERLIN image of the polarization of methanol masers around the outflow Cepheus A HW2 75 AU 14/21

15 Field mapping: Cepheus A HW 2 MERLIN image of the polarization of methanol masers around the outflow Cepheus A HW2 75 AU Submm dust polarization (Curran & Chrysostomou 7) 15/21

16 3D view of the Cepheus A disc Cepheus A HW2 (Vlemmings et al. MNRAS 21) 16/21

17 Infall motions consistent with current models of magnetic accretion Model comparison T=1 K T= K Banerjee & Pudritz 7 17/21

18 DECLINATION (J) Methanol vs. large scale dust W51 IRS2 d W51 e2 DECLINATION (J) RIGHT ASCENSION (J) RIGHT ASCENSION (J) Remarkable agreement between methanol linear polarization EVPAs and those measured from dust polarization (Surcis et al. 212, overlay on Tang et al. ) 18/21

19 Overview of sources al.: EVN group:magneticfield IRAS In Table 4, named as I I , we report all the identified 6.7-GHz CH 3 OH maser features. In particular, they are associated with the source G N and they can be divided in two groups (A and B). Group A is composed of 2 ed around H maser xdensity color. A larization ionp l,are 19/21

20 Magnetic field and outflows Apparent preferred orientation of the outflow along the magnetic field Misalignment <32 degrees <1% chance of random alignment (Surcis et al. 213) 2/21

21 Conclusions Significant Zeeman splitting detected in 33/ GHz methanol maser sources associated with high-mass star formation Corresponds to B =23 ± 5 mg in the methanol region (n H2 ~ cm -3 ) Larger than B crit ~12 mg dynamically important Average field ~6 times higher than average 1.6 GHz OH maser field Interferometric methanol maser observations probe large scale (and possibly 3D) magnetic field structure close to massive protostars Magnetic field regulated infall on Cepheus A HW2 disk Preferentially aligned with outflow direction 21/21