[PCG27-10] X-ray induced chemistry for water and related molecules in low-mass protostar envelopes
キーワード：宇宙化学、分子、原始星、スノーライン、水、X線
Water has been used to study dynamical properties of star-forming regions, and it is also one of key molecules in chemical evolutions. Recent water line observations toward several low-mass protostars suggested low water abundances in the inner warm envelopes. Water destruction by strong X-ray fluxes may influence in these regions, but detailed processes, including molecules holding oxygen instead, have not yet understood.
In our study, we calculated the chemical evolutions of low-mass Class 0 protostar envelopes using the detailed gas-grain chemical reaction network including X-ray induced chemical reactions, and investigated the dependences of water and related molecule’s abundances on X-ray radiation fields.
If the central protostars have higher X-ray luminosities (LX>1030 erg s−1), water gas abundances become higher (up tp x(H2O)~10−8−10−7) just outside the water snowline (T<100 K), compared with the values (x(H2O)∼10−10) in the cases of lower X-ray luminosities (LX<1030 erg s−1). Inside the water snowline (T>100 K), in the cases of lower X-ray luminosities, water gas molecules maintain the high abundances of 10-4, and they are considered to be the dominant oxygen carrier with CO. On the other hand, in the cases of higher X-ray luminosities, water gas abundances become much smaller just inside the water snowline (T∼100−250 K, below to x(H2O)∼10−8−10−7) and in the innermost hot regions (T∼250 K, x(H2O)∼10−6). In these cases, molecular and atomic oxygen abundances reach around 10−4 within the water snowline. In addition, some other water related molecules, such as HCO+ and CH3OH, are also affected by X-ray radiation fields. These X-ray effects are larger in the envelope models with lower number densities. Current and future molecular line observations for protostars (e.g., ALMA) will access the regions where such X-ray induced chemistry is important.
In our study, we calculated the chemical evolutions of low-mass Class 0 protostar envelopes using the detailed gas-grain chemical reaction network including X-ray induced chemical reactions, and investigated the dependences of water and related molecule’s abundances on X-ray radiation fields.
If the central protostars have higher X-ray luminosities (LX>1030 erg s−1), water gas abundances become higher (up tp x(H2O)~10−8−10−7) just outside the water snowline (T<100 K), compared with the values (x(H2O)∼10−10) in the cases of lower X-ray luminosities (LX<1030 erg s−1). Inside the water snowline (T>100 K), in the cases of lower X-ray luminosities, water gas molecules maintain the high abundances of 10-4, and they are considered to be the dominant oxygen carrier with CO. On the other hand, in the cases of higher X-ray luminosities, water gas abundances become much smaller just inside the water snowline (T∼100−250 K, below to x(H2O)∼10−8−10−7) and in the innermost hot regions (T∼250 K, x(H2O)∼10−6). In these cases, molecular and atomic oxygen abundances reach around 10−4 within the water snowline. In addition, some other water related molecules, such as HCO+ and CH3OH, are also affected by X-ray radiation fields. These X-ray effects are larger in the envelope models with lower number densities. Current and future molecular line observations for protostars (e.g., ALMA) will access the regions where such X-ray induced chemistry is important.