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					    Detection of the 205 m [NII] Line from the Carina Nebula
                             G. J. Stacey1, T. E. Oberst1, A. Loehr2, S. C. Parshley1, T. Nikola1, N. F. H. Tothill2, J. I. Harnett2,
                                                        A. P. Lane2, A. A. Stark2, and C. R. Tucker3
                                                      1Department of Astronomy, Cornell University
                                                      2Harvard-Smithsonian Center for Astrophysics
                                                                      3Cardiff University


                                                                                                               Summary
                                                  We report the first detection of the [NII] 205 m (1.46 THz) fine structure line from the ground using a direct
                                                  detection spectrometer. The line was detected and mapped in emission from the Carina star formation region
                                                  in the Galaxy using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter
                                                  Telescope and Remote Observatory (AST/RO) at the South Pole. The [NII] line is an important coolant for the
                                                  diffuse ionized ISM, and (together with the 122 m [NII] line) is an excellent probe of gas density. The [NII]
                                                  205 m line emission peaks at the Carina II HII region, and its strength indicates low density (n ~ 30 cm-3) low
                                                  ionization state gas envelopes the HII region. When compared with the ISO [CII] observations of this region,
                                                  we find that a surprising large fraction (~ 50%) of the [CII] line emission arises from this low density ionized                                         Aurora over the South Pole August 22, 2005
The SPIFI instrument mounted on the Nasmyth
                                                  gas, a result with implications for [CII] studies of photodissociation regions (PDRs). The detection of this line                                        (photo by Stephen Parshley)
arm of the AST/RO telescope.
                                                  demonstrates the utility of Antarctic sites for THz spectroscopy.



                  The Carina Nebula                                                                                                                                                                             Discussion
 The Carina Nebula is a large star formation complex in the                              The [NII] maps entailed ~ 20 pointings, or >
southern Galactic plane subtending nearly 4  4 or ~ 150                                                                                                                           Prior ISO Results
                                                                                          400 spectra covering the two regions:
150 pc at d = 2.7 kpc                                                                                                                                                                  ISO/LWS mapped much of the same regions in the [OIII] and
                                                                                           Carina I: G287.4-0.6                                                                      [NII] 122 m lines (Mizutani, Onaka, and Shibai, 2002)
 The optical nebula is dominated by the Carina I and                                                                      Figure 1. 5  5
                                                                                           Carina II: G287.6-0.6          pixel SPIFI                                                 O++ takes 35 eV to form so it traces gas exposed to UV light
Carina II HII regions which together form an arrow -head                                                                                                footprint (red
shaped nebulosity (Figure 1)                                                                                                                                                          from high mass stars. From the [OIII] lines, they find the highly
                                                                                                                                                        square) and
 The Carina I and Carina II HII regions appear to be                                                                                                   regions mapped
                                                                                                                                                                                      ionized gas has two components:
excited by the Trumpler 14 and Trumpler 16 star clusters –                                                                                              (blue outlines)                    Relatively dense (ne ~ 100 to 350 cm-3) component
although the star clusters are offset by ~ 2 to 4 pc with                                                                                               superposed on                     associated with the Carina I and II HII regions
respect to the free-free continuum peaks
                                                                                                                                                        Digital Sky                        Relatively diffuse (ne < 100 cm-3) “halo” component
                                                                                                                                                        Survey image                      encompassing the entire complex with a diameter of ~ 30 pc
 The Trumpler 16 and 14 star clusters contain ~ 36 early                                                                                               (from Mizutani et
type stars, including the peculiar IR bright star  Carina, a                                                                                           al. 2002).
member of Trumpler 16 cluster
                                                                                                                                                                                     The [NII] Line Ratio
                                                                                                                                                                                      The ISO 122 m [NII] to SPIFI 205 m line ratio yields the
                                                                                                                                                                                     density of the low ionization gas (Figure 5, blue line and circle)
                    The [NII] Lines                                                                                                                                                      The ratio is ~ 1.2  ne ~ 30 cm-3
 Far-IR [NII] fine-structure lines arise from ground state                                                                                                                              Even with a 50% calibration uncertainty, it is clear that the
term (Figure 2)                                                                                                                                                                         [NII] lines (tracing low ionization gas) arise from a very low
                                                                                         [NII]                                                                     [NII] contours:
 Levels are collisionally excited (by electrons),                  1S
                                                                      0                                                                                            fraction of          density medium: 10 < ne < 100 cm-3
energetically accessible, and the lines are typically optically                                                                                                    peak (0.51 K)      Therefore, the “halo” of emission seen in the [OII] lines also
thin so that [NII] lines are important coolants for the ionized                                                                                                    0.25              contains gas in lower ionization states
                                                                               5755 Å                              5’
gas                                                                                                                                                                0.38               This low ionization, diffuse gas is similar to the warm ionized
                                                                                                                                                                   0.50
     [NII] 205 m is an important coolant on Galactic scales:                                                                                                     0.63
                                                                                                                                                                                     medium in the Galaxy as a whole
                                                                                                  3063 Å
    ~ 3rd brightest far-IR/submm line observed by FIRAS on                                                                                                         0.75
    COBE – after [CII] and [NII] 122 m (Wright et al. 1991)                                                                                                       0.88
                                                                    1D
     In ionization-bounded HII regions [NII] 205 m can be           2                                                                                            1.00
    a proxy for the Lyman continuum flux
                                                                                                                                                                                     [NII] and [CII]
                                                                           6583 Å                                                                                                     The [CII] line was mapped with ISO LWS and the KAO, and is
     The [NII] 122/205 m line ratio yields cloud density
     The [NII]/[CII] 158 m line ratio constrains fraction of                           122 m                                                                                      both bright and widespread over the Carina Nebula (Mizutani,
                                                                                 6548 Å           205 m                                                                             Onaka, and Shibai, 2004, Brooks et al. 2003)
    [CII] from ionized gas
                                                                                                                    Carinae                                                          In galaxies the [CII] line emission predominantly arises from the
 While the optical fine structure lines are probes of electron     3P
                                                                           2
                                                                           1                                                                                                         warm dense photodissociation regions (PDRs) on the far-UV
temperature, the far-IR lines are extinction-free probes of gas                                            Figure 3. SPIFI [NII] 205 m contours (blue &
                                                                           0                                                                                                         exposed surfaces of molecular clouds (cf. Stacey et al. 1991)
density                                                                                                    red) superposed on (black) 21 cm continuum
                                                                        Figure 2. N+ Energy level diagram. contours (Retallack, Flux Ratios
                                                                                                                        Line 1983)                                                        -- However, [CII] can also arise from low density ionized gas
                                                                                                                                                                                      With an ionization potential of 14.5 eV, the 205 m [NII] line
                                                                        0.55                                                     10
                                                                                                                                                                                     only arises from ionized gas regions
                       Observations                                                             [NII] 205 n
                                                                                                                                  9
                                                                                                                                                                                      Since the [NII] 205 m line has a nearly identical critical density
                                                                        0.45                                                      8
The Carina Nebula was observed in the 205.178 m [NII]                                                                                                                               for thermalization as [CII] in ionized regions (~ 50 cm-3), the
                                                                                                 Carina II
                                                                                                                                                                                     [CII]/[NII] 205 m line intensity ratio yields the fraction of the
                                                                                                                                  7
                                                                        0.35                                                          [CII]/[NII] 205
line with SPIFI (Bradford et al. 2002) on the 1.7 m AST/RO
                                                                                                                    Line Ratio




                                                                                                                                  6
                                                                  TMB




telescope (Stark et al. 2001) at the South Pole. The data                                                                         5
                                                                                                                                                                                     observed [CII] that arises from the ionized medium – subject to
                                                                        0.25
presented here were obtained on 23 to 25 August 2005, near                                                                        4
                                                                                                                                                                                     relative C+ to N+ abundance estimates
the middle of our 30 day run. We mapped the region                      0.15                                                      3
                                                                                                                                                                                      Figure 5 displays the predicted [CII]/[NII] line intensity ratio (red
outlined in red in Figure 1 in weather that was unusually poor                                                                    2
                                                                                                                                                                                     line) for ionized gas as a function of gas density
for the South Pole site, with zenith transmission ~ 7.6%.
                                                                        0.05
                                                                                                                                  1
                                                                                                                                                        [NII]: 122/205                The observed value (red dot) of 8 means that 4/8 ~ 50% of the
                                                                                                                                  0                                                  observed [CII] line emission arises from the ionized medium
                                                                        -0.05
                                                                             -223 -131    -39    53    145   237
                                                                                                                                      0         10          1000       100000         This is a much larger fraction than the 10 to 25 % commonly
SPIFI-AST/RO Parameters                                                                                                                       Ionized Gas Density
                                                                                    VLSR(km/sec)                                                                                     assumed in neutral gas studies of external galaxies, and has
   Beam size:          ~ 54” (FWHM)                                                                               Figure 5. Theoretical, and observed [NII]                         implications for the modeling of photodissociation regions
   Field of view:      ~ 325”  325”                              Figure 4. [NII] 205 m line detected            122/205 m line ratio (blue line and dot) and                     including gas density and temperature, and the strength of
   Resolving Power:    R  /~ 5000, or v ~ 60 km/sec          from G287.57-0.59 (Carina II).                  [CII] 158 m/ [NII] 205 m line ratio (red line
                                                                                                                                                                                     stellar radiation fields in these studies
   Sensitivity:         NEP ~ 3.4  10-15 W/Hz1/2 (front end)                                                     and dot) as a functions of ionized gas density.
                              Trec(DSB) ~ 150 K
                                                                                                      Acknowledgements
                         Results                                         This work was supported by NSF grants OPP-0094605, OPP-                                                                               References
 The map of Figure 3 contains only about 20% of our data                0338149, and NASA grant NNG05GK70H. We are indebted to the                                                  Bradford et al. Ap. Opt. 41, 2561 (2002)
                                                                                                                                                                                     Brooks, K.J. et al. A&A, 412, 751 (2003)
 The [NII] line appears in emission over most of the region             Cardiff group under P. A. R. Ade for their excellent filters, to the
                                                                                                                                                                                     Brooks, K.J., Storey, J.W.V., & Whiteoak, J.B. MNRAS 327, 46 (2001)
 The observed line emission largely follows the free-free               GSFC group (S.H. Moseley, D. J. Benford, & J. G. Staguhn) for their
                                                                                                                                                                                     Mizutani, Onaka, & Shibai A&A 382, 610 (2002)
radio contours (Retallack, 1983)                                         excellent bolometers, and to J.W. Kooi who set up LO’s for our                                              Mizutani, Onaka, & Shibai A&A 423, 579 (2004)
 The line peaks at ~ 0.51 K on the position of the Carina II            frequency calibration at the pole. We also thank the many people                                            Retallack, D.S. MNRAS 204, 669 (1983)
radio continuum peak (G287.57-0.59, Figure 4)                            who have contributed to the success of SPIFI both at AST/RO and                                             Stacey et al. ApJ 373, 423 (1991)
 The line velocity centroid well matches that of the radio              the JCMT including, and especially C. M. Bradford, A. D. Bolatto,                                           Stark, A.A. et al., PASP, 115, 567 (2001)
recombination lines (Brooks, Storey, & Whiteoak, 2001)                   J.A. Davidson, & M. L. Savage                                                                               Wright, E.L. et al. ApJ 381, 200 (1991)

				
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