Wind bubbles and HII regions around slowly moving O stars

Transcription

1 Wind bubbles and HII regions around slowly moving O stars Jonathan Mackey Argelander-Institut für Astronomie (aifa), Uni. Bonn Collaborators:! Vasilii Gvaramadze! (Lomonosov Moscow State University, Russia)! Shazrene Mohamed!(SAAO, Cape Town)! Norbert Langer! (aifa, Bonn) AG Meeting 2014, Bamberg, Splinter session J: The Interstellar Medium

2 Outline HII regions and wind bubbles around O stars Observations of the HII region RCW120 2D simulations of slowly-moving O stars X-ray emission from bubble Infrared emission (preliminary)

3 Stellar Wind Bubbles Wind bubbles around main sequence O stars are rarely observed. Only(?) example is the Bubble Nebula, NGC Star has strong mass loss (>10-6 M per year) Bubble appears to be young (expanding at 25 km/s, age <50,000 years). Not detected in X-rays. Problem: energy input in wind ~10 36 erg/s, but X-ray luminosity < erg/s. NGC 602 in the SMC Moore et al. (2002,AJ,124,3313)

4 Somebody should run some models of this! Moore et al. (2002,AJ,124,3313) NGC 602 in the SMC

5 Indirect Evidence Of Wind Bubbles? N49 observed with Spitzer. Everett & Churchwell (2010) interpreted the emission as mixed wind and ISM gas, but this does not explain the central cavity. Ochsendorf+(2014) suggest radiation pressure excludes dust from region near central star (dynamically decoupled from gas). Could cavity inside 24 μm emission be a wind bubble instead? It should exist! Watson et al. (2008,ApJ,681,1341) N49 24 μm (red), 8 μm (green), 4.5 μm (blue)

6 Another young HII region: RCW 120. Deharveng+(2009): Blue = H-alpha, Red=24μm, Green = 8μm

7 RCW 120 HII Region Young H II region in the Southern Sky, d 1.35+/-0.3 kpc (Zavagno+,2007).! Ionised by O6-8 star CD mass M 29 M (Martins+,2010); - age is 0 3 Myr.! Dynamical age of H II region is ~ Myr if star is static (Arthur+, 2011).! Shown here is a false-colour image in H-alpha and infrared.! Apparently isolated, although there are other low-mass stars in the region.! Did it form in isolation, or is it a runaway?! ( or, is there an unseen cluster?) Deharveng+(2009): Blue = H-alpha, Red=24μm, Green = 8μm

8 Isolated Massive Stars Runaways, dissolved clusters/associations, or formed in isolation? (1/4 of all O stars) Important constraint on theories, if massive stars always form in clusters or in isolation. How can we tell if a star is a runaway? Sometimes easy, sometime not so easy. Vela X-1 (Kaper+,1997) RCW 158, from Gvaramadze & Bomans (2008,A&A,490,1071) SMC isolated HII regions (Oey+,2013)

9 RCW 120 HII Region Shown here are Spitzer images at 8μm (left) and 24μm (right), with the ionising star shown as the cross. Asymmetry could be signature of motion how fast is reasonable? Wind bubble? Does 24μm emission show a bow shock? (cf. Ochsendorf+2014)

10 H II Region Properties Size: 3.4x3.8 pc (still quite circular). 0.9 pc from upstream edge, 2.8 pc from downstream edge. Shell mass ~2000 M. Shell seems most unstable upstream. Shell is almost closed (in projection). At 1.35 kpc distance, 100 = 0.65 pc Anderson+ (2010): 250 μm (red) and 100 μm (green) Herschel data, and 24 μm Spitzer data (blue).

11 Simulations 2D Axisymmetric grid simulations using the pion radiation-mhd code (Mackey & Lim,2010,2011, Mackey 2012). Non-equilibrium ionization of H; non-equilibrium heating and cooling. Star at origin; uniform ISM flowing past star; wind imposed near the origin. Star based on CD : Q 0 =3x10 48 s -1, T eff =37.5kK, Q FUV =7.5x10 48 s -1 Stellar wind (models): Mdot = M yr 1, v w =2000 km/s. ISM density n H =3000 cm -3 (7.0x10-21 g. cm -3 ).

12 Simulations Neutral/molecular gas cooling based on Henney+(2009) from CLOUDY calibrations. Atomic H cooling: calculated explicitly based on H + fraction (Frank & Mellema,1994, Hummer 1994). Photoionised gas: cooling from [C+/++],[N+/++],[O+/++], (Henney +2009). Hot gas: CIE from metals, plus Bremsstrahlung (Wiersma+,2009). Minimum temperature set for numerical convenience, to artificially stabilise the HII region shell.

13 4 km/s Model

14 4 km/s Model

15 8 km/s Model

16 16 km/s Model: gas density

17 Total shell mass vs. time Shell properties Shell mass at z>0 vs. time Shell mass is independent of velocity, but upstream mass depends on velocity and is still accumulating. In terms of mass, all models can match observed shell mass of 2000 M, given enough time. For V * >~ 20 km/s, upstream shell can no longer form completely (Raga et al., 1997; Mackey et al. 2013), and is unstable.

18 X-ray emission from wind bubble Total radiated energy above threshold T Total emission, according to cooling curve of simulation. Red line = wind mechanical luminosity: 0.5 Mdot v 2 = 2x10 35 erg/s. Very little energy is radiated by hot gas. It mixes with photoionized gas and radiates by line emission. Only ~ % of the wind energy is radiated by gas with T>10 6 K.

19 X-ray emission from wind bubble X-ray luminosity Unabsorbed X-ray emission: XSPEC tables for solar metallicity gas. Red line = wind mechanical luminosity: 0.5 Mdot v 2 = 2x10 35 erg/s. Very little energy is radiated in X-rays (changes very little over time). Only ~ % of the wind energy is radiated by gas with T>10 6 K.

20 Infrared Emission Preliminary Synthetic infrared images from the simulations using the TORUS code. Preliminary 24 um (green) and 70 um (red) image.

21 Jonathan Mackey! AG2014: ISM splinter Conclusions HII regions are much more spherical than wind bubbles. Slow-moving stars have aspherical wind bubbles. Wind bubble remains ~10-25% the volume of the HII region. X-ray emission is soft and weak, difficult to detect. Wind bubble loses >99% of its energy by turbulent mixing. Mid-IR arcs in HII regions could show wind bubble s edge. Axis ratio, size, and mass of RCW 120 shell indicate that space velocity is <~4 km/s; star must have formed there! Is the ionising star really isolated? Are there A or B stars? Does the region have an unusual initial mass function?