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
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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. -8-
telescopes simultaneously to simulate the size of a large single-dish telescope with a loss in
sensitivity.
1.4.2. Radio Interferometry
Interferometry involves measuring wavefronts from a source at different positions
and then combining them to produce interference fringes. Several antennas at different
positions detect wavefronts which are converted to voltages, passed through amplifiers
and multiplied together. The multiplication of the voltages takes place in a correlator
which yields an output known as the complex visibility containing information on the
instantaneous a. The complex visibility varies as the source moves across the sky and the
projection of the baseline on the sky can be graphically represented on a (u-v) plane which
is the Fourier Transform of the image brightness distribution. The visibility is proportional
to a component of the Fourier Transform of the source brightness distribution, therefore the brightness distribution of the source can be determined from the Fourier transform of the observed visibilities. Due to the finite, and often small number of baselines, the interferometer samples the (u,v) plane in a non-uniform manner, so to recover the source brightness distribution you need extensive sampling of the (u,v) plane.
1.4.3. The Australia Telescope Compact Array (ATCA)
The ATCA is a 6 element East-West radio interferometer with 22-m antennas operating at cm and mm wavelengths, up to 3 mm. The telescope resides in the Paul Wild
Observatory about 500 km north-west of Sydney. Five of the array telescopes are situated along a three kilometer railway track while the sixth telescope is located three kilometers to the west, providing 15 baselines. ATCA uses Earth Rotation Synthesis to sample a series of - 9 -
concentric ellipses in the (u,v) plane. For example, an interferometer composed of several antennas lying along an east-west line which are tracking a radio source, will observe the source rotating about the pole. Over a 12-hour period the relative rotation will be 180' producing half an ellipse in the (u,v) plane.
1.5. Hydrogen Recombination Lines
In the ionized gas of HII regions, electrons are captured by ions and become bound in an energy level with a large principal quantum number. The electron will then cascade down from level to level, releasing electromagnetic energy with each jump. The energy is released in a series of lines known as recombination lines producing a unique energy spectrum for each molecule. These recombination lines contain gas density and gas velocity information. The average radial velocity of a cloud can be found from the Doppler frequency shift of an emission line. Broadening of the line by pressure effects gives a measure of the gas density while Doppler effects on the line shapes and center frequencies yields velocity fields of the gas.
Transitions that occur at large principal quantum numbers emit radiation in the radio region of the electromagnetic spectrum. Line frequencies can be calculated from the Rydberg formula:
v = RcZ 2 [ (2) n2 (n + An)2 where c is the speed of light, R is the Rydberg constant, Z is the effective charge of the nucleus, n is the lower principal quantum number, and An is the change in n. Recombination lines are identified by their atomic species, the lower principal quantum number and their change in quantum number. Hydrogen recombination lines begin with an H, followed by the lower principal quantum number. A An=1 corresponds to the symbol a and An=_2 correspond to 3, etc. For example, the recombination line designated H107a 10
refers to the transition in hydrogen between the 108 level to the 107 level.
In this paper we investigate the ionized gas associated with RCW38. Studies at radio wavelengths of the continuum and hydrogen recombination line yield information on the velocity, morphology and excitation parameters of the ionized gas in the HII region. We have made observations of 1666 MHz continuum emission, 4800 MHz continuum emission
and the H107a recombination line. We compare these to data taken at other wavelengths, specifically infrared and X-ray wavelengths.
2. Observations
2.1. 1666 MHz Continuum Observations and Reduction
RCW38 was observed at two frequencies, 1666 MHz and 4800 MHz. We observed RCW 38 at 1666 MHz with the ATCA in its 1.5D and 6.OC configurations 1996 May and June. On May 10, 1996 a 13.7 hr track was obtained with the 1.5D configuration of the array, while on June 15, 1996 a 10.97 hr track was obtained with the 6.0C configuration.
Observations with ATCA require flux, phase and bandpass calibration which is achieved by observing calibrator sources. The flux calibration source is typically a strong point source whose intensity and position are well known. The phase calibrator provides an absolute reference position to calibrate the phase of the visibilities. It is typically a point source located nearby in the sky to the target source so the measured phase variations are a close representation of the variations seen by the target. The primary flux calibrator used was PKS B1934-638 with an assumed flux of 14.16 Jy, and the phase calibrator was PKS B0823-500. The visibility data was processed with the MIRIAD package. Bandpass solutions were determined for PKS B1934-638 and copied to PKS B0823-500. The time dependent antenna based gains were determined for PKS B0823-500 and bad data was - 11 flagged. The flux scale was then bootstrapped to PKS B1934-638. The gain solutions were copied to RCW 38 and the continuum was constructed from the line-free channels in the visibility data set, and the line and continuum visibilities split. Very little data needed to be flagged and interference was basically non-existent. The data for the two configurations were calibrated separately and later combined during the mapping stage. Maps were created by Fourier transforming the visibilities using the MIRIAD CLEAN algorithm, and restoring the images with a 10"x 10"beam. The observing parameters are given in Table 1 and the u-v coverage can be seen in Figure 3.
XX 1.6680 GHz
CD
till J - /
0 C\2
-20 0 20 u (k X)
Fig. 3.- The 1666 MHz u-v coverage for the June 15, 1996 and May 10, 1996 ATCA observing run. - 12 -
Table 1. Observing Parameters for ATCA 1666 MHz Continuum Observations
Parameter Value
Observing Dates 10 May & 15 June 1996 Observing Time 13.7 & 10.97 hr Primary Beam 25' Synthesized Beam 9'!80 x 8'"30 in P.A. 170
Phase center ..... a20O0 = 0 8 h5 8 m 4 8s 8 2
62000= -47030'58/!8 Spectral channels 4096 Bandwidth ...... 4 MHz
Polarizations .... 1 Size conversion ... 1' ~ 0.5 pc - 13 -
2.2. 4800 MHz Continuum Observations and Reduction
We observed RCW 38 at 4800 MHz over four days in April 1999. Tracks were taken April 21, April 27, April 28, and April 30 for 5.23 hrs, 5.66 hrs, 2.80 hrs and 9.85 hrs respectively. All data was taken in the 1.5C configuration. The u-v coverage for these observations was not as complete as the 1666 MHz observations since only one configuration wwas used for all observing runs. The data was calibrated and CLEANed with the same procedure as was used for the 1666 MHz continuum. The observing parameters are given in Table 2 and the u-v coverage is plotted in Figure 4.
2.3. H107a Recombination Line Observations and Reduction
In addition to the continuum observations at 4800 MHz on April 21, 1999, the H107a recombination line at 5293 MHz was observed. The data was calibrated in the same manner as the continuum. To form the H107o recombination line emission maps, the continuum was subtracted from the line data. A data cube was formed with each two dimensional plane being R.A. and Dec, and each plane of a different velocity. The planes were incremented by 4 kms-' (0.929 MHz) ranging from -60 to 60 km s-. Each channel image was deconvolved using the Miriad CLEAN algorithm and restored with a beam size of 10"x 10". The observing parameters are given in Table 3 and the u-v coverage is plotted in Figure 5. 14
Table 2. Observing Parameters for ATCA 4800 MHz Continuum Observations
Parameter Value
Observing Dates 21, 27, 28, & 30 April 1999 Observing Time 5.23, 5.66, 2.80, & 9.85 hr Primary Beam ... 10' Synthesised Beam 8''23 x 3'"94 in P.A. 6'4
Phase center ..... a2000 =08 h58m4882
J2000= -47O30'58'I8 Spectral channels 13 Bandwidth ...... 104 MHz Polarizations ..... 4
Size conversion .. 1' ~ 0.5 pc - 15 -
XX 4.8480 GHz
0 - N - N - -- N N -. --- -~ N
-- --- ~- N ~ x / ,- .;,~\
/ N -p-, / N ~ -- ~. - N *~N. -~-- - - N ~ ~7- N N N ~ - C, N N - -~ - N - I I I -50 0 50
u (kx)
Fig. 4.- The 4800 MHz u-v coverage for the April 21, 27, 28, and 30, 1999 ATCA observing run.
XX rcw38.5293 5.3010 GHz
0
/ N N
0 N~
I ii I '' ' I I II I I~ )
0 I C\2 N \ \ \\\ ~ "I N N - \ N 7 N
-50 0 50
u (kX)
Fig. 5.- The u-v coverage for the April 21, 1999 ATCA observations of the H107a recom- bination line. Note the lack of coverage in comparison to the continuum observations. The recombination line was only observed for 5.66 total hrs. 16
Table 3. Observing Parameters for ATCA H107a Observations
Parameter Value
Transitions ...... Ha 5293.65 MHz Observing Dates ... 21 April 1999
Observing Time ... 5.66, 2.80 & 9.85 hr
Primary Beam .... 10' Synthesised Beam . 14'.'31 x 3''77 in P.A. -0'4
Phase center ...... a 2000 = 08h58m48s82
62000 =-47-30'58'.8 Spectral channels .. 257
Channel Separation 0.063 MHz (3.57 km s- 1 ) Bandwidth ...... 16.2 MHz (917.6 km s--) Polarizations ...... 4
Size conversion .... 1' - 0.5 pc 17 -
30
3000
30
0-1 0 0 31 00 -n- C7 0 30
3200
-473230
08h59m 10s 05s 59mQ0s R.A. (J2000)
Fig. 6.- 4800 MHz greyscale continuum image with 1666 MHz contours. The white cross indicates the position of IRSI and the white star indicates the position of IRS2. The image was restored with a beam size of 10" by 10" shown in the lower left corner. - 18
3. Results and Analysis
3.1. Continuum Emission
3.1.1. Morphology
The overall continuum emission is concentrated in a ring-like structure with a diameter of approximately 1 arcminute corresponding to a ring about half a parsec in diameter. The center of the ring shows less emission and also roughly corresponds to the location of IRS2, an 05 star previously identified by Frogel and Persson (1974) and the position for the source was determined by Smith et al. (1999). The emission extends slightly south-west but is mainly concentrated in the ring around IRS2.
The overall morphology and features of RCW38 matches in both the 4800 MHz continuum image and the 1666 MHz continuum image. Many of the features agree with previous observations at different frequecies which we will discuss later in this paper. The similarities between the continuum images at different frequecies can be seen in Figure 6. Figure 6 shows 1666 MHz contours overlaid on a grey-scale 4800 MHz continuum.
The ring itself contains several interesting features. Figure 7. shows a grey-scale of the 1666 MHz continuum image with several selected regions, boxed and numbered. The ring does not exhibit uniform emission but instead contains knots of bright emission components. The most striking feature in the ring is the bright ridge located in the western portion of the ring which corresponds to Region 1 in Figure 6. This ridge has previously been observed at higher frequencies and roughly coincides with the peak in the emission at 10pm known as IRSI (Frogel and Persson, 1974, Smith et al. 1999). The ridge is about 0.2 pc by 0.15 pc and is the brightest feature in both the 4800 MHz continuum as well as the 1666 MHz continuum. - 19 -
I
30 -
0
U)13 -4731
I I I I 08h59m 1Os 08 06s 04s R.A.
Fig. 7.- 4800 MHz greyscale continuum image with selected regions where integrated line profiles have been obtained. The black star to the left of region 1 is the bright 05 star IRS2.
Region 2 in Figure 7 is a moderate emission area. It is not nearly as bright as Region 1, and also slightly smaller. Region 4 is slightly brighter than its surrounding area while regions 3 and 5 are bright, relatively large knots at the northern and southern edges of the ring. In Tables 4 and 5 we list flux, optical depth, emission measure and ionizing photons per second which were calculated for each of the regions shown in Figure 7.
We compared the radio continuum at 4800 MHz and 1666 MHz to previous observations in the K-band and X-ray. Figure 8 illustrates the comparision of the radio continuum emission to a K-band image of RCW38 taken with the VLT (J. Alves et al. 2003, in preparation). The bright ridge in the radio continuum emission which we have designated as Region 1 corresponds to a similar ridge in the K-band image. The cavity in the center of the radio emission can be seen to contain the brightest K-band source in the field which 20
corresponds to the location of IRS2. It also appears that the hole in the radio emission coincides with a similar hole in the extended K-band emission. No obvious K-band features can be seen that correlates to regions 3, 4, and 5, but the overall structure is similar. The infrared image still shows a hollow structure although it is less ring-like than that seen in the radio continuum emission.
Diffuse X-ray emission of RCW 38 was previously studied by Wolk et al. (2002). The X-ray emission follows a different trend than the K-band or radio emission. Figure 9 shows a comparison of the diffuse X-ray emission to the 4800 MHz continuum map. The peak of the diffuse X-ray contours is centered on IRS2 and decreases relatively symmetrically outward. The ridge containing IRS1 does not appear to have any counterpart in the diffuse X-ray emission although the contours do appear to kink around the ridge. Instead, the X-ray contours closely follow the edge of the cavity in the radio continuum image, filling in the areas corresponding to little radio emission.
3.2. Recombination Line
3.2.1. Morphology
Figure 10 shows channel contours of the H107a emission from RCW 38 overlaid on the 4800 MHz continuum grey-scale image. The figure covers a velocity range from -36 to 24 kms- 1 where H107a emission is seen. The line emission does not follow the general shape of the continuum emission. There is no clear ring structure in the line emission. Instead, it appears that much of the line emission is contained within the continuum ring. the line emission shows two to three bright spots near the ring and emission in the hole of the radio ring, mostly extending from the bright ridge to the east.
Three features that match in both the continuum emission and the line emission are - 21 -
30 00"
30 0 0 0
0
3100
-4731 30
08 h59m10s 05s R.A. (J2000)
Fig. 8.- K-band greyscale image with 4800 MHz radio continuum contours.
the bright ridge in the area designated region 1, the bright spot in the northern edge of the radio ring designated region 3 and the bright spot designated region 4. The bright ridge along the eastern edge of the ring is detected across all velocities. The maximum emission occurs approximately near the cloud's rest velocity and shows no significant velocity trends. The shape of the ridge is slightly different in the line than the continuum. The bright ridge source extends toward the center of the radio ring and is not confined to the edge of the ring. - 22 -
30
3000 N
30 Oi-
01- 0 0 - - 3100 0
-0 30
- - 3200
-473230
08 59m1 05s 59m00s R.A. (J2000)
Fig. 9.- 4800 MHz greyscale continuum image with diffuse X-ray contours.
Emission from region 3 is clearly evident between -24 kms- 1 and about 8 kms- 1. A slight peak in the recombination line emission is seen in each of the channels within this velocity range.
Emission from region 4 is evident between -20 kms~ 1 and 0 kms- 1. Again, a slight peak in the emission is seen in this range. The other bright spots in the continuum ring
(regions 2 and 5) do not have line emission counterparts and the hole in the continuum does - 23 -
not correspond to a hole in the line emission.
Figure 11 is an grey-scale 4800 MHz continuum image with H107a contours integrated over all velocities. The image illustrates the strong emission from region 1, as well as the absence of a hole in the emission or a clear ring structure. The line emission from regions 3 and 4 can also be seen in the shape of the contours.
30 30'00 30 3100
-47'31 30 -6.0 -- 32.0 -28.0 -24.0 30 3000 30
0 3100 0 0 -4731 30 6 -- 20.0 -- 1 6.0 -12.0 -7-8.0 0) 30 0 3000 30 3100
-4731 30 -- 4.0 T 00 4.0 ~~8.0 30 3000 30 3100 .31130 1 -47 - 12.0 ~~16.0 ~~20.0 ~724.0
08 59mlOs 05s 59m00s 05s 59m'00" 05s 59m00 053 59m00 R.A. (J2000)
Fig. 10.- A sequence of H107a line images of RCW 38 for velocities ranging from -36 to 24 km s-1, with a velocity resolution of 4 km s-1. The beam size is 10"x 10". - 24 -
I I I
30
3000
0 0 0 30
0 0- 3100
I- I * I .
-4731 30
08 59m 14s 12s 108 088 068 048 02s 59m00s 58s R.A. (J2000)
Fig. 11.- Velocity integrated H107oz line contours over 4800 MHz continuum grey-scale. The beamsize is 10"by 10", shown in the lower left corner. The white star corresponds to the location of IRS2.
3.2.2. Recombination Line Profiles, Parameters and Derived Quantities
A set of line profiles has been obtained by integrating the emission over the selected regions shown in Figure 7. Table 5 lists the results of Gaussian fits to each of the profiles, including peak intensity, central velocity, and linewidth. The line-to-continuum ratio and the electron temperature were calculated from the observed measurements.
The line profiles from the 5 regions are shown in Figure 12. Each profile contains a - 25
component centered near the rest velocity and the linewidth values range from 25 to 39 km
s_-. The profiles exhibit possible double peaked features but poor signal to noise does not allow us to confidently identify these features.
The electron temperature was calculated using the gaussian fits and assuming local thermodynamic equilibrium via eq. [22] from Roelfsema and Goss (1992):
Te (6943v 1 1 L 1 1 )0. 7 (3) ( L/C),AV I + Y+ where L/C is the line-to-continuum ratio, v given in GHz, the linewidth AV in kms-1, and the helium abundance Y+ was taken to be a typical value of 0.1 (De Pree et al. 1999). The calculated values range from 5200 K to 7500 K which is comparable to the single value of 7500 K derived from previous H109a observations using the Parks telescope with a beam size of 4'4 (Caswell and Hayes, 1987).
3.2.3. Continuum Parameters
Parameters measured from the 5 regions in the continuum emission are listed in Table 4. The parameters include the 4800 MHz continuum optical depth, emission measure, and the ionizing photon flux per second (all calculated following Molinari et al. 1998). The electron temperature was calculated from the H107a recombination lin. The spectral type of the ionizing star of this region was estimated from the ionizing photon flux using the results from Panagia (1973) and Molinari et al. (1998).
The optical depth for each region was calculated using the equation:
T = -In I - (4) where T is the beam brightness temperature of the continuum measured in units of K and 26
Table 4. Results of the Gaussian fits to the H107a line profiles from the selected regions within RCW 38. Parameters include peak intensity (P), central velocity (V), linewidth (AV), line-to-continuum ratio (L/C) and electron temperature (Te).
Region P V AV L/C Te Jy/beam km s' km s 1 K
1 0.324 0.016 1.214 1.286 32 2 0.047 0.013 6400 700 2 0.103 0.016 0.094 3.779 28 5 0.136 0.016 5200 300 3 0.213 0.016 -0.752 1.716 26 2 0.106 0.012 6900 700 4 0.153 0.016 1.671 2.350 25 3 0.123 0.014 6200 500 5 0.099 0.016 5.723 4.692 39 7 0.065 0.007 7500 1100 27 -
0 0 CO to
crDS CY)
0 CO0 C (U CC I \ 0 gz*9 0 ~I* 0 93,0 0/
~ I
\ i / "I C2 0
- -
o 2 Co Si \I)
02 0 TO 93*O 0 / uire~q/Af LO CC) 0 0 Uo11f
CO CO K'' CrO 00 - 0 0 Co Q 0 0@ CO0 Co TO 93*0 0 T
Fig. 12.- Selected regions within RCW 38 and their corresponding line profiles.
/ 28
Te is the electron temperature in units of K. The emission measure (EM) was given by:
EM = (5) 8.235 x 10- 2Te-1. 3 5 V_ 2 (1 where EM is in units of pc cm- 6 and the frequency v/ is expressed in GHz. The photon flux of the Lyman continuum (NLy) has been estimated as follows:
1 4 5 NLy(s- ) =4.76 X 1048Sint 2v0. -0. (6) where St is the integrated flux density in units of Jy, d is the distance in kpc, and v is in units of GHz. The results are given in Table 4.
We find region 1 to be more optically thick than the rest of the regions. We also find that if the ionizing source for the region is a single star, it would be of the spectral type 05/06 (Molinari et al. 1998), consistent with previous determinations by Frogel and Persson (1974) and Smith et al. (1999) for IRS2. Frogel and Persson determined the spectral type through studies of NIR colours and fluxes while Smith et al. used the strengths of mid-infrared ionization lines. All three observations agree with the spectral type of IRS2 being an 05 star.
4. Summary
The cm radio emission in RCW 38 is concentrated in a ring-like structure roughly centered on the 05 star IRS2. It appears IRS2 is the ionizing source for the region and winds from the star are blowing out the hydrogen gas in its vicinity. This would indicate the ring is actually a shell formed by winds originating from IRS2. The double peaked feature in the line profiles which cannot be accurately resolved with our current signal to noise, is a signature of an expanding shell. Future observations with better u-v coverage could possibly confirm that the ring is in fact an expanding shell. - 29 -
Table 5. Total integrated fluxes and physical parameters derived from the 4.8-GHz continuum
images. Physical parameters include optical depth T, emission measure EM, and the total
number of ionizing photons per second NLy.
Region Speak STotal T EM NLy Jy/beam Jy/beam 106 pc cm- 6 104 8s-1
1 0.0672 0.0135 3.944 0.0135 0.0175 0.004 0.781088 0.18 1.23 t 0.06 2 0.0363 0.0053 1.552 0.0053 0.0116 0.002 0.390773 0.06 0.53 0.02 3 0.0387 0.0055 3.58 0.0055 0.0092 0.002 0.459355 0.08 1.08 0.05 4 0.0350 0.0037 2.16 0.0037 0.0093 0.001 0.399008 0.05 0.69 0.03 5 0.0370 0.0056 3.28 0.0056 0.0081 0.002 0.453131 0.10 0.95 0.06 RCW 38 31.9 0.07 30
It also appears that much of the recombination line emission we observed is found within the radio ring. Possibly, the central area has a lower temperature where the electrons are able to recombine which results in the emission that we detect inside the ring. Also, the coincidence of the peak in the X-ray emission with the hole in the radio emission suggests a strong interaction between the X-ray gas and the ionized hydrogen gas. The ring of ionized hydrogen gas could possibly be confining the X-ray gas, although not completely.
We also find that a single 05 star can account for the bulk of the ionizing photons. This confirms previous estimates and observations by Frogel & Persson (1974) and Smith et al. (1999). This estimate gives further credibility to the idea that IRS2 is an 05 star and the ionizing source of the region.
Studies of the region at different wavelengths indicates an interesting interaction between molecular and ionized gas. Further studies could reveal the nature of the radio ring we have observed and its effects on its surrounding environment. - 31
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This manuscript was prepared with the AAS LATEX macros v5.0.