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H107a Recombination-Line Emission, 4800-MHz and 1666-MHz Continuum Emission in the HII Region RCW38 Miquela Vigil MIT Dept. of Earth. Atmospheric, and Planetary Science Nav 21, 2003 1, ,r-eL 0O73 The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part Copyright @ Massachusetts Institute of Technology. All rights reserved. ARCH IVES MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEP 28 2017 LIBRARIES Signature redacted Signature of Author- ina-T -r- -. da.t i\iquela C. Vioil Certified by _S ignature redacted Richard P.Binzel Thesis Supervisor Bignature redacted Tyler Bourke Thesis Advisor Signature redacted Certified by-------- Scott Wolk Thesis Advisor Sia ture redacted Certified by------ - J. Brian Evans Department Thesis Coordinator The author hereby grants to MIT pem.ission to reproduce and to distribu-1 publicly paper and elfectronic copos of Vi; thesis document in whole or in parl in any medium now known or hereaftercreated. 2 ABSTRACT We present results from observations of H107a recombination-line emission and the related 4800 MHz continuum emission of the HII region RCW 38 using the Australia Telescope Compact Array. We find the continuum emission to be concentrated in a ring-like structure with the 05 star, IRS2, approximately centered in the cavity within the ring. The temperature of the ionized gas ranges from 5200 to 7500 K and the emission is optically thin. The H107a line emission appears to be confined within the continuum ring. We also find the continuum ring to encircle the peak in the diffuse X-ray gas. The radio continuum emission matches closely to NIR observations with a bright western ridge containing the peak in the 10pm emission known as IRS1 (Frogel et al. 1974) apparent in both observations. From calculations of continuum and line parameters, we estimate the spectral type of the ionizing source for the region to be an 05/06 star which is consistent with the spectral type of IRS2. 1. Introduction 1.1. Star Formation and the Interstellar Medium Star formation occurs in the dense cores of molecular clouds. Young stellar clusters embedded within these molecular clouds provide key information, like the inital mass function, on the process of star formation in galaxies. Embedded clusters are either fully or partially surrounded by interstellar gas and dust which causes them to be difficult to detect at optical wavelengths and easiest to detect in the infrared and longer wavelengths, e.g., millimeter and centimeter wavelengths. The model for the formation of low mass stars (Shu, Adams, & Lizano 1987; Shu et -3- al. 1993) begins with the central region of a dense core where the core slowly condenses until the central region becomes unstable. In this state, the thermal pressure is the only force counteracting the self-gravity of the core. The core then undergoes free-fall collapse. After collapse, three evolutionary stages follow: 1.) an accretion stage which involves a protostar and circumstellar disk surrounded by in-falling gas and dust; 2.) a stage in which the protostar transfers momentum, angular and linear, to its surroundings through jets and molecular outflows; and 3.) a stage where the protostar becomes optically visible and then enters the zero age main sequence, decreasing its luminosity. Massive star formation proves to be a more dynamical process than low mass star formation. First, the time scales for formation and evolution of massive stars are shorter than low mass stars. They begin burning hydrogen and will reach main sequence even before they have stopped accreting matter. Second, the massive protostar affects its surroundings at an earlier stage. Third, massive stars produce significant output of UV photons and strong winds which can have appreciable effects on the surrounding medium. The UV photons will ionize the surrounding gas while the winds can push out gas and dust from the surroundings. This region of ionized hydrogen is known as an HII region. The effects on the surroundings can possibly trigger further star formation in nearby regions, but may also curb star formation in the immediate vicinity due to the excavation of dust and gas by winds from newly formed stars. The surrounding environment of embedded clusters changes with the evolution of the cluster. The youngest clusters are usually partially or fully embedded within the cold dense molecular material of the core, while the older, more evolved clusters are surrounded by hot dusty regions of ionized hydrogen. Studies of these clusters at different stages in their evolution allow us to construct a picture of the process of star formation. 4- 1.2. HII Regions Hydrogen atoms in the interstellar medium can be ionized by ultraviolet radiation from young, hot stars creating regions called HII regions. HII regions are created by massive, bright and short-lived stars which are as massive as 20-100 solar masses but are more luminous than 10000 Suns. They die in supernovae explosions and their lifetimes are typically on the order of 10 million years. Designated as OB stars, these bright stars are typical tracers of star forming regions. Most stars form in clusters but OB stars are particularly usefull because they are extremely bright making it possible to detect them from great distances. The gas temperature of HII regions is typically between 5000-10000K and the average number densities are in the range of 100-10000 particles per cubic centimeter (Murdin 2001). Compared to terrestrial conditions, HII regions are very hot and dilute regions. Two types of radio emission that are detected from HII regions are continuum emission and recombination line emission. Ionized HII regions are filled with hydrogen ions. Free electrons in the interstellar medium are attracted to these ions resulting in their paths being deflected toward these regions. This deflection results in emission of radiation known as free-free emission or continuum emission. The second type of emission is produced when these free electrons are captured by a positive atom producing an atom in an exciting state. The atom loses its energy by cascading down energy levels releasing radiation. Each transition releases radiation known as recombination line emission. A commonly used method to study recombination line emission from embedded HII regions is observations at cm wavelengths where many transitions occur. -5- 1.3. RCW38 The southern HII region RCW 38 (Rodgers, Campbell and Whiteoak, 1960) is a young, massive embedded stellar cluster containing an 05 star, IRS2 (Frogel and Persson 1974), clustered within hundreds of lower mass stars. RCW 38 is located in the region of the Gum nebula and the Vela supernova remnant, at a distance of 1.7 kpc, which makes it one of the closest star forming regions containing an early 0-type star. From previous NIR observations (Frogel and Persson 1974; J. Alves et al. 2003 in preparation) RCW 38 appears to be a blister HII region lying just inside the edge of a giant molecular cloud. NIR Very Large Telescope (VLT) images reveal an embedded cluster surrounding IRS2 which can be seen in Figure 2 (Alves et al. 2003). Previous studies of RCW38 by the Chandra X-ray Observatory by Wolk et al. (2002), find diffuse X-ray emission in the region which they model as synchrotron emission. In Figure 1 the peak of the diffuse X-ray emission corresponds to a cavity in the diffuse infrared emission. Further studies of the region in the radio can help determine whether the emission from the region is thermal or nonthermal synchrotron emission as well as what the interactions are between the ionized gas and the X-ray plasma. 1.4. Radio Telescopes and the Australia Telescope Compact Array 1.4.1. Single Aperture Synthesis The intensities, distribution, and position of cosmic radio sources can be obtained through observations by radio telescopes. Radio telescopes can be either a single steerable antenna or arrays of dishes. The Half Power Beam Width (HPBW) is referred to as the beamwidth (0b), and can be estimated for any telescope observing at a particular -6- Fig. 1.- The image on the left was taken by the Two Micron All Sky Survey (2MASS) project and shows a 10'by 10'region containing RCW 38. The image on the right is a NIR Very Large Telescope (VLT) image about 2.5'by 2.5'focusing in on RCW 38. The bright ridge to the west contains the peak in the 10pm emission known as IRS1 (Frogel et al. 1974). The bright source to the east of the ridge is known as IRS2 and believed to be an 05 star (Frogel et al. 1974). wavelength from: 3480' D D is the diameter of the aperture of the telescope in wavelengths. For example, a 22-m antenna operating at 6 cm (5 GHz) results in a beamwidth of about 10'. Resolution increases with decreasing beamwidth therefore improving the resolution requires increasing the size of the aperture. Obviously there is a practical limit to the size of a single radio telescope. A different technique called interferometry can be employed to improve resolution without building impractically large single dish telescopes, but instead using multiple radio -7- 30 30,00" 0 0 3100 -47*.31'30"-- 08 h59m 109 053 59mO0S R.A. (J2000) Fig. 2.- Diffuse X-ray contours overlaid on a K-band image taken with the VLT (J.Alves et al. 2002 in preparation). Contours represent 0.16, 0.4, 0.8, 1.6, 3 and 5 X-ray counts per pixel. Coordinates are J2000. The peak in the diffuse X-ray emission coincides with a cavity in the diffuse K-band emission. There also appears to be an interation between the diffuse X-ray gas and the bright ridge containing IRS2 as can be seen in the deformation of the contours around the ridge.

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