Centro de Radioastronomía y Astrofísica, UNAM

1. Centro de Radioastronomía y Astrofísica, UNAM

2. Massive Star Formation Luis F. Rodriguez, CRyA, UNAM Morelia, México • Introduction • The paradigm for low-mass star formation • The search for jets and disks in massive young stars • Other mechanisms? • Conclusions

3. LOW MASS STAR FORMATION • Fragmentation of cloud • Gravitational contraction • Accretion and ejection • Formation of disk • Residual disk • Formation of planets (Shu, Adams & Lizano 1987)

4. L1551 IRS5: Binary system with disks. VLA 7mm Lim & Takakuwa (2006) Minus third component and jet contribution

5. L1551 IRS5: Binary system with jets VLA 3.6 cm Rodriguez et al. (2003)

6. On a much larger scale, there is evidence of circumbinary structures (White et al. 2006) _______ CH3OH _______ HCN _______ CO _______ CO Onsala data

7. Evidence for circumbinary disk also from Takakuwa et al. (2004)

11. Formation of Massive Stars • With great advances achieved in our understanding of low mass star formation, it is tempting to think of high mass star formation simply as an extension of low mass star formation. • That is, assume that the accretion into the star continues until we have a massive object. • However…

12. Some problems with extending the picture of low-mass star formation to massive stars: • Radiation pressure acting on dust grains can become large enough to reverse the infall of matter: • Fgrav = GM*m/r2 • Frad = Ls/4pr2c • Above 10 Msun radiation pressure could reverse infall

13. So, how do stars with M*>10M form? • Accretion: • Need to reduce effective s, e.g., by having very high Macc • Reduce the effective luminosity by making the radiation field anisotropic • Form massive stars through collisions of intermediate-mass stars in clusters • May be explained by observed cluster dynamics • Possible problem with cross section for coalescence • Observational consequences of such collisions?

14. Other differences between low- and high-mass star formation • Physical properties of clouds undergoing low- and high-mass star formation are different: • Massive SF: clouds are warmer, larger, more massive, mainly located in spiral arms; high mass stars form in clusters and associations • Low-mass SF: form in a cooler population of clouds throughout the Galactic disk, as well as GMCs, not necessarily in clusters • Massive protostars luminous but rare and remote • Ionization phenomena associated with massive SF: UCHII regions • Different environments observed has led to the suggestion that different mechanisms (or modes) apply to low- and high-mass SF

15. One observational approach has been the search for disks and jets in massive forming stars. • We will talk mostly of a handful of sources that we have studied with collaborators from many institutions.

17. HH 1-2 VLA 1 VLA 1

18. What are the thermal jets? • (Partially) ionized, collimated outflows that emanate from young stars. • Detectable as weak free-free sources. • They are believed to be the “base” of the large scale outflow phenomena like the bipolar outflows and HH systems. • They are almost always found in the case of low-mass protostars, but rarely in high-mass protostars.

19. Why are thermal jets rare to find in association with high mass protostars? • Different formation mechanism? • Confusion from bright HII regions in region? • Stellar multiplicity a serious problem. • Molecular outflows (large scale) are, however, relatively frequent. Timescale problem?

20. Timescale problem? • Bipolar outflows in low mass protostars have dynamic ages of 104 years, much shorter than the K-H time of the jet/disk stage of 106 years. • In contrast, bipolar outflows in high mass protostar have dynamic ages of 105 years, longer than the K-H time of the jet/disk stage of 104 years. • That is, in high mass protostars, the jet may turn off and the large scale outflow will still persist as a fossil for a relatively long time.

21. An example: IRAS 12091-6129 B0/O8.5 according to distance ambiguity. About 100 solar masses in the molecular outflow. No reported jet.

22. Correlations suggest same phenomenon…

23. HH 80-81 (GGD27) in L291 dark cloud Distance 1.7 kpc (Rodríguez et al. 1980), Luminosity: 2 x 104 LSol Star: B0.5 ZAMS

24. Highly collimated jet with extension of 5.3 pc (11´ ) (Martí, Rodríguez & Reipurth 1993)

25. CO (blue) NO clear evidence of a disk in HH 80-81 CO (red) CS (2-1) torus (Nobeyama 45m, 36¨ resolution)

26. H2O maser Thermal Jet Gómez et al. 1995

27. Marti et al. (1998) analyzed the thermal jet over several years.

28. Derive velocities for knots of 500 km/s.

29. Observations (1999): VLA NH3 (1,1) y (2,2) Resolution ~4´´ at 1.3 cm NH3 (1,1) 15´´ Continuum jet at 3.6 cm H2O

30. 7 mm emission observed with the VLA at a resolution of 0.4´´ Dust ~ +2.6 Free-free S~ +0.2 Gómez et al. 2003

31. Continuum flux grows rapidly as S ~2.6 suggesting dust emission. Assuming dust emission is opticallty thin and temperature between 50-100 K, < total mass of 3.7 – 1.8 Msol is derived following Beckwith & Sargent (1991). 1.4 mm 3.5 mm BIMA (resolutions of 7.4 y 19´´, respectively)

32. One of the best cases is Cep A HW2 (Patel et al. 2005) Dust (colors) and molecular (CH3CN, in green contours) emissions perpendicular to bipolar jet. Radius of disk = 330 AU Mass of disk = 1-8 MSUN Mass of star = 15 MSUN SMA and VLA data

33. Position-velocity map across major axis of disk implies M = 19 +-5 MSUN

34. Sequence of images of radio jet at 3.6 cm Curiel et al. (2006)

35. The controversy… • Brogan et al. (2007) and Comito et al. (2007) argue that not a disk but multiple condensations: Outflowing 3.6 cm knots and 875 microns SMA image also shown.

36. The counterarguments: Torrelles et al. (2007)

37. Jimenez-Serra et al. (2007)

38. Torrelles et al. (2007) argue for a continuous structure that can be imaged with various tracers.

39. There are, however, problems with the poor signal-to-noise of the data and non-Keplerian rotation, that Jimenez-Serra et al. (2007) attribute to extreme youth of object.

40. O STARS • IRAS 16547-4247 (Garay et al. 2003)

41. IRAS 16457-4742 At a distance of 2.9 kpc, it has a bolometric luminosity of 62,000 solar luminosities, equivalent to an O8 ZAMS star.

42. Garay et al. (2003) found millimeter continuum emission (dust) and a triple source in the centimeter range. Core has 1,000 solar masses. Data from SEST (mm) and ATCA (cm)

43. Australia Telescope Compact Array

44. Data from ATCA, the components are not clearly resolved.

45. VLA images of IRAS 16547-4247

46. The wide wings in the molecular lines suggest the presence of high velocity gas in a bipolar outflow. This has been recently confirmed. Data from SEST

47. The outflow carries about 100 solar masses of gas (most from ambient cloud) and has characteristics of being driven by a very luminous object.

48. Molecular hydrogen (2.12 micronss) tracing the bipolar outflow (Brooks et al. 2003) Data from ISAAC in the VLT

49. VLA data at 2 cm The central source is resolved as an elongated object In particular, the position angle of 165 +- 2 degrees aligns well with the lobes. We observe a dependence of angular size with frequency characteristic of ionized outflows.

50. However, the axis of the jet misses the lobes. We are investigating this problem (common in triple sources of this type).