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Sh 2-173, Cassiopeia

Sh 2-173, Cassiopeia

530 mm f/5 21.3 hours, October 2013, Photograph

Sh 2-173 — The HII region Sh 2-173 resides in the Perseus sprial arm at an estimated distance of 2.5 +/- 0.5 kpc and is detected in optical and radio continuum images, and as a cavity in HI emissions. Seven hot stars are projected in the field of Sh 2-173, including only one O-type star. The small triangles indicate the position of the seven stars. The amount of energetic photons emitted by the O-type star is enough to keep the region ionized and its dust heated. The HII region is partially bordered by a photodissociation region, which, in turn, is partially encircled by a molecular structure detected in CO emissions. A population of 46 young stellar objects (YSO) candidates has been identified, projected on to the molecular cloud.

Sh 2-173 is located in a dense edge of a large (~5°) HI shell, GSH 117.8+1.5-3.5, together with several other HII regions. This morphology suggests that Sh 2-173 may be part of a hierarchical system of three generations. In this scenario, the large HI shell, which probably originated due to the action of the Cas OB5 association, would have triggered the formation of Sh 2-173, which, in turn, is triggering new stars in its surrounding molecular cloud. Sh 2-173 has an estimated dynamical age of 0.6-1.0 Myr. The large HI shell has an estimated dynamical age of 5 +/- 1 Myr.

Sh 2-173 has a linear radius of 9.5 +/- 2.0 pc, an ionized mass of 480 +/- 250 Msolar, and an rms electron density of 6 +/-1 cm⁻³. The heated dust counterpart to the ionized hydrogen region has a mass of 1.7 +/- 0.7 Msolar and a temperature of 30 +/- 5 K. The partially encircling molecular structure has a mean area of 80 +/- 10 pc, a molecular mass of 5.8 +/- 2.0 kMsolar, and a volume density of 520 +/- 210 cm⁻³. From Cichowolski et al., "Unveiling the birth and evolution of the H II region Sh2-173", Monthly Notices of the Royal Astronomical Society, 394:900-915, 2009.

Ou 2 — The planetary nebula Ou 2 was discovered by amateur astronomer Nicolas Outters. The nebula spectrum is dominated by the green [OIII] doublet, a high intensity He II 4680 line, and Hα. This implies a very high excitation, and a very hot central star, which is visible on the Second Palomar Sky Survey B image. From Acker et al., "Discovery of new faint northern galactic planetary nebula", Revista Mexicana de Astronomía y Astrofísica, 48:223-233, 2012.

NGC 103 — The open cluster NGC 103 has an estimated age of 130 Myr, a distance of 3.0 kpc, and a color excess E(B-V) of 0.41. From de La Fuentes Marcos et al., "The Cassiopeia Perseus open cluster family", New Astronomy, 14(2):180-195, 2009, Feb.

NGC 136 — The open cluster NGC 136 has an estimate age of 250 Myr, a distance of 5.2 +/- 0.4 kpc, and a color excess E(B-V) of 0.43 +/- 0.03. From Glushkova et al., "Photometry of the poorly studied galactic open star clusters King 13, King 18, King 19, King 20, NGC 136, and NGC 7245", Astronomy Letters, 36(1):14-26, 2010, Jan.

VX Cas — VX Cas is a Herbig Ae pre-main-sequence star, a young star of spectral type A0e. The star is still embedded in a gas-dust envelope and is surrounded by a circumstellar disk. The star exists in the formation stage and is not yet burning hydrogen in its center. The photometric and polarimetric variability of the star suggest that circumstellar gas and dust clouds can enter the close vicinity of the star and be distrupted there. From Shakhovskoi et al., "Photometric and polarimetric activity of the Herbig Ae star VX Cas", Astronomy Reports, 47(4):301-311, 2003, April.

Coordinates: RA 0h 24.8m, Dec +62° 2’, PA -0° 44’
Field of view: 1° 56’ x 1° 28’
Image scale: 4.2” / pixel

Date: October 2013
Location: Kofa Mountains, Arizona
Exposure: 32 x 40 minutes (21.3 hours), binned 2 x 2 at -20° C

Zenith SQM: 21.5 mag / "²
Guiding error: 0.6" rms
Star FWHM: 4.1", histogram
Star eccentricity: 0.38
Camera noise: 15 e- rms
Background noise: 26 e- rms
Subframe location: 261 e-
Subframe scale: 51 e-

Telescope: Takahashi FSQ-106EDX 530 mm f/5
Camera: Quantum Scientific Imaging 683wsg
Filter: Astrodon H-alpha, 3 nm bandwidth
Focuser: Finger Lakes Instrumentation Atlas, Quality Thermistor DirecTemp DTU6024C, Spike-a Bahtinov Mask
Guider: Santa Barbara Instrument Group ST-i
Flats: Alnitak Astrosystems Flat-Man
USB: Icron Ranger 2122, Moxa UPort 404
Mount: Takahashi EM-400 Temma2M
Acquisition: Diffraction Limited MaxIm DL, CCDAstro Temma, IHRP Platform, ASCOM Platform
Processing: Pleiades Astrophoto PixInsight, MathWorks MATLAB, IHRP Platform