Optical Polarization and Infrared Spectrum of a Possible Protostar in a Reflection Nebula

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 77; no. 1; pp. 14 - 17
• Main Authors: Ney, E. P., Hatfield, B. F., Gehrz, R. D.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01.01.1980; National Acad Sciences
• Subjects: Astronomical objects; Carbon stars; H II regions; Main sequence stars; Mathematical vectors; Nebulae; Observatories; Physical Sciences: Astronomy; Polarized light; Reflection nebulae; Stars
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.77.1.14

The star HD 200775 illuminates the reflection nebula NGC 7023. This star is one of a small group of Ae and Be stars associated with nebulosity and proposed by Herbig as candidates for newly formed stars of moderately large mass. The system is a part of a large dust complex more than a degree in extent. Structural features in the nebulosity can be traced very near to the star (several arc seconds away) and some of these structures are highly polarized in the sense expected for scattered light from the central object. Although the star is classified as an early spectral type (B2-B8) of moderate luminosity (class IV or V), its energy distribution does not resemble a black body characteristic of a stellar photosphere. Instead, the radiated flux has the appearance of free free radiation (thermal bremsstrahlung) usually considered to arise in astrophysical situations involving optically thin plasmas of ions and free electrons. Our observations are consistent with the belief that HD 200775 is a protostar and that the surrounding material is the cocoon from which it formed. The complex polarized structures probably originate in material ejected by radiation pressure and plasma processes as the star rose in luminosity after its gravitational collapse. The reflection nebula NGC 7023 is illuminated by a luminous embedded star HD 200775. We present observations supporting the view that this object is a newly formed star, which has a morphology suggesting the ejection of plasma and dust of the cocoon from which it formed as the star begins its nuclear burning stage or perhaps even during its late stages of gravitational collapse. The star has a thermal bremsstrahlung spectrum indicating a compact HII region of diameter less than 4 arc sec and optically polarized structural features extending to hundreds of thousands of astronomical units from the star. Although the mass of the star is probably less than 10 solar masses, the dust and gas nebula immediately surrounding it has a mass of 10-100 solar masses. Detailed studies of this nebula should contribute to our understanding of the earliest stages of stellar evolution.