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Jets and Disks in Young Stellar Objects

Jets and Disks in Young Stellar Objects. Introduction Jets and disks in low-mass young stellar objects The search for jets and disks in massive young stars Other mechanisms for massive star formation? Conclusions and prospects. Luis F. Rodriguez, CRyA, UNAM Morelia, México.

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Jets and Disks in Young Stellar Objects

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  1. Jets and Disks in Young Stellar Objects • Introduction • Jets and disks in low-mass young stellar objects • The search for jets and disks in massive young stars • Other mechanisms for massive star formation? • Conclusions and prospects Luis F. Rodriguez, CRyA, UNAM Morelia, México

  2. Molecular Cloud

  3. Bipolar Molecular Outflows Molecular gas moving away from regions of star formation. Snell, Loren, & Plambeck (1980) Rodríguez, Ho, & Moran (1980)

  4. Moving Herbig-Haro Objects HH 1 HH 2

  5. Thermal Jets: the Ionized, Collimated Ejecta from Young Stars Optical; Herbig & Jones (1981) Radio 6-cm; Pravdo et al. (1986)

  6. What are the thermal jets? • (Partially) ionized, collimated outflows that emanate from young stars. • Ionization seems to have a shock origin. • 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.

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

  8. Jets and Disks in the Radio • I will now concentrate on observations made at radio (cm, mm, and sub-mm) wavelengths with interferometers. • Why radio? The early stages of star formation are heavily obscured. • Why interferometers? We want to understand the small scales of the phenomenon. • There is, however, a vast body of observations and theory that addresses the many aspects of star formation (McKee & Ostriker 2007, Zinnecker & Yorke 2007).

  9. JET: removes excess angular momentum and magnetic flux, produces outflows and HH objects. Free-free source at cm waves. DISK: allows accretion, planets may form from it. Thermal dust emission at mm waves.

  10. DISK JET Rodriguez et al. 2008 Gomez et al. (2003) VLA continuum observations at several frequencies reveal characteristic spectrum jet+disk.

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

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

  13. Two stars, two disks, two jets…

  14. Itoh et al. (2000) Subaru image showing the two jets emanating from dark core. Green color traces [Fe II] lines in J and H infrared bands.

  15. Low Mass Star Formation • Believed to take place via disks and jets. • However, data in general limited in angular resolution and with little kinematical evidence.

  16. Evidence for a rotating disk… DG Tau: CO observations by Testi et al. (2002) made with OVRO. A Keplerian mass of 0.67 solar masses is estimated.

  17. HH 30 HST images by Watson et al. (2008)

  18. Anglada et al. (2007) proposed that the jet precession implies a binary system. Guilloteau et al. (2008) used the IRAM interferometer at 1.3 mm and find evidence for a circumbinary disk. R(outer) = 250 AU R(inner) = 37 AU Semiaxis of binary = 15 AU; Pichardo et al. (2005)

  19. Summary of jets and disks in low mass forming stars Disks: Diameters of 30 to 300 AU. Masses of orden 0.1 solar masses. Some may actually be circumbinary rings. Jets: Mass loss rates of order 10-6 solar masses per year at velocities of 100-200 km s-1.

  20. Formation of Massive Stars • With good 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, there are differences between low-mass and high-mass star formation.

  21. 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

  22. So, how do stars with M*>10M form? • Accretion: • Need to reduce effective s, e.g., by having very high mass accretion rates (McKee & Tan 2002). • Reduce the effective luminosity by making the radiation field anisotropic. • Form massive stars through collisions of intermediate-mass stars in clusters (Bally & Zinnecker 2005) • Problem with cross section for coalescence. • Observational consequences of such collisions?

  23. 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) may apply to low- and high-mass SF

  24. A very similar cartoon… Credit: C. Purcell

  25. One observational approach has been the search for disks and jets in massive forming stars. • However, only of a handful of sources have been studied in detail.

  26. 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. Are we seeing “fossil” molecular outflows?

  27. “Fossil” molecular outflows? • 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.

  28. W51e2: An outflow with an HCHII region (Keto and Klassen (2009) H53alpha extent (contours) and velocity gradient (color) of HCHII at center. Color: High velocity CO (2-1) Contours: 1 mm emission

  29. 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

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

  31. H2O maser Thermal Jet Gómez et al. 1995

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

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

  34. 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

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

  36. 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

  37. Disk or multiple condensations? • 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.

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

  39. 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.

  40. 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)

  41. VLA images of IRAS 16547-4247

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

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

  44. OK, so we believe we have a jet • What about infalling gas and in particular, a disk?

  45. Some of the line emission from single dish (20”) observations show profiles characteristic of large scale infall. You need much larger angular resolution to detect a disk.

  46. The SubMillimeter Array

  47. Velocity gradient in SO2 (colors) suggests total mass of 20 to 40 solar masses and a radius of 1,000 AU for the disk (Franco-Hernandez et al. 2007). Most massive young star known with jets, disk, and large scale infall.

  48. Also water masers suggest rotating structure with same direction as SO2, but at much smaller scales. Keplerian mass also about 30 Msun

  49. Summary of jets and disks in high mass forming stars Disks: Diameters of 300 to 1000 AU. Masses of orden a few solar masses. Some may actually be quite clumpy. Jets: Mass loss rates of order 10-5 solar masses per year at velocities of 200-500 km s-1.

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