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Download SHELL H II REGIONS IN NGC 6334
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Centro de Radioastronomía y Astrofísica, UNAM Campus Morelia Proper Motions of Stars and Gas in our Galaxy Luis F. Rodríguez Centro de Radioastronomía y Astrofísica, UNAM, Campus Morelia Radio results of proper motions of stars and gas and in our Galaxy that allow in some cases the determination of masses and distances and in others a much deeper understanding of the origin, age and evolution of the source studied. Will not include extragalactic results, neither results involving maser lines. The proper motions discussed here are obtained from continuum (thermal and nonthermal) observations of galactic sources, moving at nonrelativistic speeds. The radial velocity of a source can be obtained from a single measurement and the application of the Doppler effect. In contrast, the plane-of-the-sky velocity requires of at least two measurements, as separated in time as possible. The effect is usually very small… • A body moving at the respectable speed of 1,000 km/s in the pane of the sky, and located at the center of the Milky Way (at 8.5 kpc), will take 40 years to move one arc second. Definition of proper motion STELLAR MASSES • Most of the information on stellar masses comes from the study of orbital motions in binary systems, using Kepler´s third law: 4 2 a 3 mM 2 G P STELLAR MASSES • That is, if we know a, the semimajor axis of the orbit, and P, the orbital period, we can find m+M, the sum of the masses of the two stars. But, how can we study very young stars that form inside clouds with high obscuration and that cannot be studied with the usual optical and infrared techniques? • Fortunately, some young stars have radio emission that allows this type of studies. RADIO OBSERVATIONS • Remarkably, protostars can be tracked at radio wavelengths due to three processes: 1. Gyrosynchrotron from active stellar magnetosphere 2. Free-free emission from ionized outflows 3. Thermal emission from circumstellar disks No extinction. However, processes (2) and (3) produce extended sources. These emissions can or cannot be present. L1551 Ha + [SII] Devine et al. (1999) L1551 IRS5 VLA-A 2 cm Proper Motions • The large, lineal motions are due to the relative motion between the Sun and the object and they coincide with what is expected. • However, there is also relative motion between the two components, suggesting orbital motions. Total proper motions “Relative” proper motions From the observations and making the following (reasonable) assumptions: • • • • Plane of orbit near plane of the sky. Circular orbit. => M+m = 1.2 Msun; P = 260 years If in the main sequence, the luminosity of this system should be like 1 solar luminosty, but it actually has like 30 solar luminosity. • This confirms that, as expected, forming stars have a large luminosity excess, most probably as a result of accretion. IRAS 16293-2422, VLA-A, 3.5 cm, un sistema triple VLBA Very Long Baseline Interferometry • You can get amazing positional precision, 0.0001” and better. • Not always can be applied, the source has to be very compact and relatively intense (implying nonthermal processes). • With VLBI you can measure the subtle effect of the geometric parallax, that can provide accurate distances. Stellar parallax As the Earth moves in its orbit around the Sun, the nearby stars seem to change their position with respect to the remote, “fixed” stars. d=1/p d = distance to star in parsecs p = parallax angle of the star in arc seconds. You detect the combination of the elliptical motion of the parallax plus the lineal proper motion due to relative motion. T Tauri: prototype of a stellar class. Distance = 149.0 +- 0.8 parsec, the best precision achieved for this type of stars (Loinard et al. 2006). Hipparcos Why can we do better than astrometric satellites for this type of stars? Young stars are highly obscured in the optical and infrared or, if detected, they are associated with nebulosity that difficults the astrometry. Not all the stars are in bound orbits… • In the Orion Nebula we found two young massive stars that appear to have been ejected from the same point some 500 years ago (the same age of the University of Valencia). BN moves to the NW at 27+-1 km s-1. I moves to the SE at 12+-2 km s-1. Encounters of three or more bodies can produce the formation of close binaries or even mergers, with the ejection of one star and possibly of gas (Bally & Zinnecker 2005). Reipurth (2000) As a matter of fact, surrounding the BNM/KL region, there is a well known molecular outflow with an upper limit of 1,000 years for its age. It is conceivable that the molecular outflow and the ejection of BN and I happened in the same event. The energy in the outflow is about 4X1047 ergs, that can be provided by the formation of a close binary system or even a merger. Possibility of mergers is very important because it may provide alternate way to form massive stars. Nebular expansion • Herbig-Haro objects • Bulk expansion of planetary nebulae and compact H II regions. Radio HH objects can be seen to move away from central source. Velocities are indicators of the mass of the central source; one of the few ways to establish if we are dealing or not with massive stars. Finally, can you measure the expansion of relatively extended objects like planetary nebulae and HII regions? Yes, measure “bulk” motion. You can get the distance to these nebulae • 1. With spectroscopy, find expansion velocity. • 2. Measure angular expansion in the plane of the sky. • 3. Use relation between distance, velocity, and proper motion to find distance. • A recent example: M2-43 Guzmán et al, (2006) VLA observations Distance = 6.0 +1.5 kpc, largest distance determined with this technique. Most of the error comes from the modeling, not from the observations. Conclusions • As a result of great improvements in angular resolution and positional accuracy, astrometry undergoes a renaissance. • Approach that takes advantage of old archival data that can be compared with more recent observations. • Addresses key parameters in astronomy such as mass, distance, and age.