Abstract
Most low-mass stars – stars like our own Sun – form in large or small clusters of star formation. For every 100 low-mass protostars in such a star-forming region, there should be one star with a mass greater than 6 M1, thus having a luminosity greater than 500 L2. Thus, a considerable fraction of all stars were subject to external irradiation during their formation. It is therefore important to investigate the effect of irradiation from intermediate-mass young stars onto low-mass protostellar envelopes. In this thesis, the effect from such irradiation on the physics (such as the temperature) and chemistry (such as molecular abundances) in low-mass protostellar envelopes is studied.
The work studies the nearby low-mass star-forming region Corona Australis, in which a large proportion of the youngest low-mass protostars (so-called Class 0 and Class I objects) are located in a dense cloud situated near the luminous Herbig Be star
R CrA. The physics and chemistry of this region are studied with millimetre, submillimetre, and far-infrared observations using both ground- and space-based observatories.
To study the temperatures in the region, interferometric maps of several spectral lines of the molecule formaldehyde (H2CO) were used. The large-scale ( 2500 AU3) H2CO temperatures were found to be enhanced to at least 30–40 K, compared to the 10–15 K which should be found on such scales around the low-mass protostars as shown by radiative transfer models. The modelling also showed that this increase in temperature can be explained by the external irradiation from R CrA.
Through far-infrared observations performed by the Herschel Space Observatory,
the warm and hot gas (T & 100 K) in the region could be studied. No increase in the
temperature of this gas compared to in other similar sources was measured, but on the other hand, the warm gas was found on much larger scales than what has previously been seen towards low-mass protostars.
The chemistry of the region was studied using single-dish and interferometric observations. The observations showed that the sources in the R CrA cloud chemically do not resemble so-called hot corinos or warm carbon-chain chemistry sources (the previously known types of low-mass Class 0 objects as defined by their chemistry). The absence of complex organic molecules in combination with high abundances of radicals such as cyanide (CN) and hydroxyl (OH) suggest that the chemistry is dominated by radiation from R CrA. In the high-resolution interferometry data we also detect signs of a 100 AU Keplerian disc around the Class 0/I object IRS7B. The disc may be responsible for the lack of detections of complex organic molecules on the smaller scales as it may have flattened the density profile of the inner envelope.
1Solar masses; 1 M = 1:99 1030 kg.
2Solar luminosities, 1 L = 3:84 1026 W.
3Astronomical units, the average Earth-Sun distance; 1 AU = 1:496 108 km.
The work studies the nearby low-mass star-forming region Corona Australis, in which a large proportion of the youngest low-mass protostars (so-called Class 0 and Class I objects) are located in a dense cloud situated near the luminous Herbig Be star
R CrA. The physics and chemistry of this region are studied with millimetre, submillimetre, and far-infrared observations using both ground- and space-based observatories.
To study the temperatures in the region, interferometric maps of several spectral lines of the molecule formaldehyde (H2CO) were used. The large-scale ( 2500 AU3) H2CO temperatures were found to be enhanced to at least 30–40 K, compared to the 10–15 K which should be found on such scales around the low-mass protostars as shown by radiative transfer models. The modelling also showed that this increase in temperature can be explained by the external irradiation from R CrA.
Through far-infrared observations performed by the Herschel Space Observatory,
the warm and hot gas (T & 100 K) in the region could be studied. No increase in the
temperature of this gas compared to in other similar sources was measured, but on the other hand, the warm gas was found on much larger scales than what has previously been seen towards low-mass protostars.
The chemistry of the region was studied using single-dish and interferometric observations. The observations showed that the sources in the R CrA cloud chemically do not resemble so-called hot corinos or warm carbon-chain chemistry sources (the previously known types of low-mass Class 0 objects as defined by their chemistry). The absence of complex organic molecules in combination with high abundances of radicals such as cyanide (CN) and hydroxyl (OH) suggest that the chemistry is dominated by radiation from R CrA. In the high-resolution interferometry data we also detect signs of a 100 AU Keplerian disc around the Class 0/I object IRS7B. The disc may be responsible for the lack of detections of complex organic molecules on the smaller scales as it may have flattened the density profile of the inner envelope.
1Solar masses; 1 M = 1:99 1030 kg.
2Solar luminosities, 1 L = 3:84 1026 W.
3Astronomical units, the average Earth-Sun distance; 1 AU = 1:496 108 km.
| Original language | English |
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| Publisher | Natural History Museum of Denmark, Faculty of Science, University of Copenhagen |
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| Number of pages | 291 |
| Publication status | Published - 2013 |