<=== observer ===>
"HSMITH",\
"Smith, Howard A.",\
"Laboratory for Astrophysics",\
"National Air and Space Museum",\
"Smithsonian Institution",\
"",\
"",\
"Washington DC 20560",\
"United States",\
"202 357 4932",\
"202 633 8174",\
"howard@wright.nasm.edu"
<=== proposal ===>
"GCENTER",1,4,\
{"galactic structure"},\
{"LWS consortium", "Baluteau", "Cox", "Fischer", "Serra", "Stacey",\
"Spinoglio", "White"}
<=== title ===>
Spectroscopic Studies of the Galactic Center Region
<=== abstract ===>
SCIENTIFIC ABSTRACT
The center 10' (about 20 pc) of the Milky Way contains several
interesting phenomena which are not found elsewhere in the Galaxy.
Very near to the dynamical center, there is a unique non-thermal radio point
source, Sgr A*, which may well be a massive (10^6 M solar) black hole. Small
scale (1 pc) radio filaments appear to circulate about
Sgr A* (the "mini-spiral"). Sgr A* is also enveloped by both a
massive and dense stellar cluster and a rotating circumnuclear gas and dust
disk (CND). About 20 pc from Sgr A*, there are several straight
non-thermal radio filaments which extend about 30 pc perpendicular to
the Galactic plane. These non-thermal filaments appear to be linked
to the Galactic Center by a series of thermal radio arches.
Enveloping (and perhaps constraining) these large scale radio
structures are several massive molecular clouds. We propose to
investigate the physical conditions, dynamics, energetics and
ionization structure of many of these phenomena through a series of
LWS Fabry-Perot and grating studies. Besides being a unique and
interesting source in of itself, the center of the Milky Way also
provides a nearby prototype to study and compare to more distant
spiral nuclei.
OBSERVATION SUMMARY
Our highest priority is to obtain fully sampled grating spectra over a fully
sampled region enclosing the inner CND. Our map will be aligned with the CND
major axis and fully sample a region 2 beams (200") by 3 beams
(300") in extent in 24 pointings. The integration time
(total of 6.1 hours) was
chosen to ensure detection of lines at the level of 1E-15 W/m^2 over
the entire band, and ten times weaker at the more sensitive
wavelengths; the value is 100 times weaker than the currently known [OI] line
in this beam. The observations will provide information about the
ionization structure (e.g. [NII]/[NIII]), density, temperature and UV field
strength (e.g. [OI] 63/[OI] 145 - [CII]), shocks vs. UV
excitation of the molecular gas (e.g. OH/CO). In addition, this set
of full spectra will provide an important archival reference on the
Galactic Center.
Our second goal is a series of deep LWS Fabry-Perot
observations of Sgr A*. The 100" ISO beam will encompass the
mini-spiral and much of the inner CND. At 30 km s^-1 resolution, the
spectral lines will be resolved in this mode, permitting kinematic
studies and separating close line pairs (e.g. CO 162.8, OH 163.1,
163.4). In addition, these observations will better separate line
from continuum for particularly weak lines. We have selected five
intervals for detailed studies as being particularly rich in important lines -
many of which are not observable from KAO. In priority
order these are: 100-125 um (e.g., [NII], OH, CO, HD), 145-166 um (e.g., [OI],
[CII], CO, OH, NH3, HeH+), 50-65 um (e.g., [OI], [SI], [NIII], [OIII]), 78-90
um (e.g., OH, CO, H2O, [OIII]), and 178-183 um (e.g., H2O, H3O+). We have
chosen the integration time to permit detection of lines as weak
as about 2E-15 W/m^2. A total time of 7.3 hours is dedicated to this project,
with intervals covered as priorities 1,2, and 3
as tabulated below.
Our general third goal is a large scale map of the entire filaments/arches
region, from SagA* north to -28d 44m and a slice along the nonthermal radio
filaments, in 6 selected lines with the LWS Fabry-Perot. For these regions,
the ionization source is not clear, be it UV flux which has escaped
from the Sgr A* "cavity", ionization from nearby OB stars, or
"critical ionization" from caused by the high velocity movement of
molecular clouds across magnetic field lines. The grating scan maps will
provide our foundation for the CND region, and we have selected the six
specific lines to help address the ionization questions. First, the
[NII](121.9 um)/[NIII](57.3 um) line pair traces the hardness of the
UV field, and is especially sensitive when the ionization source has a
characteristic temperature of 30,000 K - the inferred value from the
vicinity of Sgr A*. The ratio therefore will indicate the relative
importance of local O stars and the Sgr A* radiation field to the
ionization. We will map this ratio over the full region in 120 fully
sampled pointings, including 20 along the nonthermal filaments.
We will also measure the [OIII] 51.8 and 88.3 um lines to
eliminate density as a variable. Finally, the [OI](63.2 um), [SI](56.3 um)
lines together with the published [CII] map (Polglitsch et al. 1991)
will constrain the shock contribution (critical ionization mechanism). For
these latter four lines we will fully sample a region with 25 beams (1250") by
2 beams (100"), not the full 120 pointings of the nitrogen maps.
Table of Mapping Parameters of Sag A:
LWS01 full scans:
24 point, fully sampled map, 4x6 beams spaced by 50";
coverage: 200"x300" aligned with the CND (p.a.~10d);
integration time: 21960 seconds
expected detection limit: 1E-15 -> 1E-16 W/m^2 (depending on lamda).
LWS04 line scans (in each of six lines as described in text):
[NII] and [NIII]; 25x4 point fully sampled (1250"x200"), aligned with
the CND and passing through the [CII] peak at 17h42m28s -28d52'00"; and
a 10x2 point fully sampled slice (500"x100") along the nonthermal
radio filaments;
integration time: 11611 seconds
expected detection limit: 1E-14 W/m^2.
[OI], [OIII x2] and [SI]; 25x2 point fully sampled (1250"x100");
aligned with the CND ridge and passing through the [CII] peak at
17h42m28s -28d52'00";
integration time: 10436 seconds
expected detection limit: 1E-14 W/m^2.
Table of Fabry-Perot LWS03 Scans on SagAW:
full spectral sampling;
100-125 um: integration time of 8248 seconds
145-166 um: integration time of 3774 seconds
50-65 um: integration time of 6384 seconds
178-183 um: integration time of 4200 seconds
78-90 um: integration time of 3686 seconds
total integration time: 26292 seconds
expected detection limit: ~<2E-15 W/m^2.
Additional details can be found in the LWS Elmau Document: "ISO:LWS
Guaranteed Time Proposals", LWS/90/P/SCI/135.00, November 1990.
<=== scientific_justification ===>
Time distribution for August launch:
Team top 40% second 30% last 30%
LWS: 30208 21769 16585
total: 30208 21769 16585
Time distribution for Spring launch:
not applicable
<=== autumn_launch_targets ===>
01, "LWS01", 1, "N", "GCENT", 17.70814, -28.98861, 1950, 0., 0., 9720,0
02, "LWS01", 1, "N", "GCENT", 17.70814, -28.98861, 1950, 0., 0., 12240,0
03, "LWS03", 1, "N", "SAGAW", 17.70814, -28.98861, 1950, 0., 0., 8248,0
04, "LWS03", 2, "N", "SAGAW", 17.70814, -28.98861, 1950, 0., 0., 10158,0
05, "LWS04", 2, "N", "GCENT", 17.70814, -28.98861, 1950, 0., 0., 11611,0
06, "LWS03", 3, "N", "SAGAW", 17.70814, -28.98861, 1950, 0., 0., 7886,0
07, "LWS04", 3, "N", "GCENT", 17.70814, -28.98861, 1950, 0., 0., 8699,0
<=== spring_launch_targets ===>