Nami Sakai

Abundant Carbon-Chain Molecules toward the Low-Mass Protostar IRAS 04368+2557 in L1527

We have detected the high-excitation lines of carbon-chain molecules such as C4H2 (J ¼ 100;10Y90;9), C4 H( N ¼ 9Y8, F1, F2), l-C3H2 (41,3Y31,2), and CH3CCH (J ¼ 5Y4, K ¼ 2) toward a low-mass star-forming region, L1527. In particular, the F1 line of C4 Hi s as strong as 1.7 K( TMB). The rotational temperature of C4H2 is determined to be 12:3 � 0:8 K, which is higher than that in TMC-1 (3.8 K). Furthermore, the column density of C4H2 is derived to be about 1/4 of that in TMC-1, indicating that carbon-chain molecules are abundant in L1527 for a star-forming region. Small mapping observations show that the C4H, C4H2, and c-C3H2 emissions are distributed from the infalling envelope to the inner part. Furthermore, we have detected the lines of C5H, HC7N, and HC9N in the 20 GHz region. Sincethecarbon-chainmolecules aregenerallydeficientinstar-forming cores, theaboveresultscannot simply beexplained by the existing chemical models. The following hypothesis is proposed. If the timescale of the prestellar collapse in L1527 were shorter than those of the other star-forming cores, the carbon-chain molecules could survive in thecentralpartofthecore.Inaddition,regenerationprocessesofthecarbon-chainmoleculesduetostarformationactivities would play an important role. Evaporation of CH4 from the grain mantles would drive the regeneration processes.Thepresentobservationsshow newchemistryinawarmanddenseregionneartheprotostars,whichisnamed ‘‘warm carbon-chain chemistry (WCCC).’’ Subject headingg ISM: abundances — ISM: individual (L1527) — ISM: molecules — stars: formation

Change in the chemical composition of infalling gas forming a disk around a protostar.

IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 au is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 au (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 au), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.

DISCOVERY OF THE SECOND WARM CARBON-CHAIN-CHEMISTRY SOURCE, IRAS15398 – 3359 IN LUPUS

We have conducted a search for carbon-chain molecules toward 16 protostars with the Mopra 22 m and Nobeyama 45 m telescopes, and have detected high excitation lines from several species, such as C4H (N = 9-8), C4H2(J = 100,10-90,9), CH3CCH(J = 5-4, K = 2), and HC5N(J = 32-31), toward the low-mass protostar, IRAS15398 – 3359 in Lupus. The C4H line is as bright as 2.4 K measured with the Nobeyama 45 m telescope. The kinetic temperature is derived to be 12.6 ± 1.5 K from the K = 1 and K = 2 lines of CH3CCH. These results indicate that the carbon-chain molecules exist in a region of warm and dense gas near the protostar. The observed features are similar to those found toward IRAS04368+2557 in L1527, which shows warm carbon-chain chemistry (WCCC). In WCCC, carbon-chain molecules are produced efficiently by the evaporation of CH4 from the grain mantles in a lukewarm region near the protostar. Our data clearly indicate that WCCC is no longer specific to L1527, but occurs in IRAS15398 – 3359. In addition, we draw attention to a remarkable contrast between WCCC and hot corino chemistry in low-mass star-forming regions. Carbon-chain molecules are deficient in hot corino sources like NGC1333 IRAS4B, whereas complex organic molecules seem to be less abundant in the WCCC sources. A possible origin for such source-to-source chemical variations is suggested to arise from the timescale of the starless-core phase in each source. If this is the case, the chemical composition provides an important clue to explore the variation of star formation processes between sources and/or molecular clouds.

A Molecular Line Observation toward Massive Clumps Associated with Infrared Dark Clouds

We have surveyed the N2H+ -->J = 1?0, HC3N -->J = 5?4, CCS -->JN = 43?32, NH3 (J, K) = (1, 1), (2, 2), (3, 3), and CH3OH -->J = 7?6 lines toward the 55 massive clumps associated with infrared dark clouds by using the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experiment 10 m telescope. The N2H+, HC3N, and NH3 lines are detected toward most of the objects. On the other hand, the CCS emission is detected toward none of the objects. The [CCS]/[N2H+] ratios are found to be mostly lower than unity even in the Spitzer 24 ?m dark objects. This suggests that most of the massive clumps are chemically more evolved than the low-mass starless cores. The CH3OH emission is detected toward 18 out of 55 objects. All the CH3OH-detected objects are associated with the Spitzer 24 ?m sources, suggesting that star formation has already started in all the CH3OH-detected objects. The velocity widths of the CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines are broader than those of N2H+ -->J = 1?0. The CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines tend to have broader line width in the MSX dark objects than in the others, the former being younger or less luminous than the latter. The origin of the broad emission is discussed in terms of the interaction between an outflow and an ambient cloud.

Detection of C6H– toward the Low-Mass Protostar IRAS 04368+2557 in L1527

We have detected the J = 7-6, 8-7, and 15-14 lines of C6H- toward a low-mass star-forming region of L1527. We have also detected the J = 15/2-13/2 and 33/2-31/2 lines of the corresponding neutral species, C6H, and the 81,8-71,7 line of C6H2 in L1527. This is the first detection of these three species in star-forming regions. The column density of C6H- is (5.8 ± 1.8) × 1010 cm-2, which is comparable to that in TMC-1, although the column density of C6H in L1527 is about 1/5 of that in TMC-1. Hence, the N(C6H-)/N(C6H) ratio is 0.093 ± 0.029, which is higher than that in TMC-1 by a factor of 4. This high anion-to-neutral ratio is discussed in terms of a simplified chemical model.

DEUTERATED MOLECULES IN WARM CARBON CHAIN CHEMISTRY: THE L1527 CASE

We have conducted millimeter-wave observations of deuterated species of various carbon-chain molecules toward a low-mass star-forming region, L1527, which shows extraordinary richness of carbon-chain molecules in a vicinity of the protostar (Warm Carbon Chain Chemistry; WCCC). We have detected the spectral lines of l-C3D, C4D, C4HD, DC3N, DC5N, and c-C3HD, where l-C3D and C4HD are detected for the first time in space. The deuterium fractionation ratios are found to be moderate (2% to 7%), although they tend to be higher than those in the starless core, TMC-1. The upper limit to the [CH2DOH]/[CH3OH] ratio is also as low as 3%. Therefore, high deuterium fractionation ratios reported for hot corino sources are not seen in L1527. The observed ratios mean that the depletion of CO onto dust grains had not proceeded far in L1527, compared to the hot corino case. This would be consistent with a short timescale of the starless core phase, as suggested for the possible origin of WCCC.

Abundance anomaly of the 13C species of CCH

Aims. We have observed the N = 1−0 lines of CCH and its 13 C isotopic species toward a cold dark cloud, TMC-1 and a star-forming region, L1527, to investigate the 13 C abundances and formation pathways of CCH. Methods. The observations have been carried out with the IRAM 30 m telescope. Results. We have successfully detected the lines of 13 CCH and C 13 CH toward the both sources and found a significant intensity difference between the two 13 C isotopic species. The [C 13 CH]/[ 13 CCH] abundance ratios are 1.6 ± 0. 4( 3σ )a nd 1.6 ± 0. 1( 3σ )f or TMC-1 and L1527, respectively. The abundance difference between C 13 CH and 13 CCH means that the two carbon atoms of CCH are not equivalent in the formation pathway. On the other hand, the [CCH]/[C 13 CH] and [CCH]/[ 13 CCH] ratios are evaluated to be larger than 170 and 250 toward TMC-1, and to be larger than 80 and 135 toward L1527, respectively. Therefore, both of the 13 C species are significantly diluted in comparison with the interstellar 12 C/ 13 C ratio of 60. The dilution is discussed in terms of a behavior of 13 Ci n

Production Pathways of CCS and CCCS Inferred from Their 13C Isotopic Species

The rotational spectral lines (JN = 32-21 and JN = 21-10) of 13CCS and C13CS have been observed toward a cold dark cloud, TMC-1. The strongest hyperfine component lines of 13CCS and C13CS (JN = 21-10, F = 5/2-3/2) have successfully been detected. The / abundance ratio is determined to be 4.2 ± 2.3 (3 σ). The / ratio is evaluated to be 230 ± 130 (3 σ), and hence, 13CCS is found to be significantly diluted. Such a difference between the 13CCS and C13CS abundances is also found in L1521E, which is a very young core with rich carbon-chain molecules. Therefore, the anomaly is not specific to TMC-1, but seems to be common for the CCS-rich clouds. Furthermore, we have also observed the J = 4-3 transition of 13CCCS and CCC34S in TMC-1 and L1521E and have found that the / ratio is larger than 8.4 (3 σ). This lower limit is considerably larger than the interstellar / ratio of 3, indicating that 13CCCS is diluted as in the case of 13CCS. These results give us strong constraints on the main pathways to produce CCS and CCCS.

A SURVEY OF MOLECULAR LINES TOWARD MASSIVE CLUMPS IN EARLY EVOLUTIONARY STAGES OF HIGH-MASS STAR FORMATION

We have observed the CH3OH J = 2–1, SiO J = 2–1, C 34 S J = 2–1, H 13 CO + J = 1–0, HN 13 C J = 1–0, CCH N = 1–0, OCS J = 8–7, and SO JN = 22–11 lines toward 20 massive clumps, including Midcourse Space Experiment (MSX )8 μm dark sources (infrared dark clouds) andMSX 8 μm sources, by using the Nobeyama Radio Observatory 45 m telescope. We have found that the velocity widths of the CH3OH and C 34 S lines are broader than those of the H 13 CO + line in the MSX dark sources. On the other hand, they are comparable to the velocity width of the H 13 CO + line in the MSX sources. In addition, the [SiO]/[H 13 CO + ] abundance ratio is found to be enhanced in the MSX dark sources in comparison with the MSX sources. These results suggest that shocks caused by interaction between an outflow and an ambient dense gas would have substantial impact on the chemical composition of the MSX dark sources. The velocity widths of the CH3OH and C 34 S lines relative to that of the H 13 CO + line as well as the [SiO]/[H 13 CO + ] abundance ratio could be used as good tools for investigating evolutionary stages of massive clumps. On the basis of the results, we discuss the chemical and physical evolution of massive clumps.

Tentative Detection of C4H− toward the Low-Mass Protostar IRAS 04368+2557 in L1527

The millimeter-wave rotational emission line (J = 9–8) of the negative ion, C4H−, has tentatively been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the IRAM 30 m telescope. The column density of C4H− is determined to be 1.1 × 1010 cm−2. The [C4H−]/[C4H] ratio is found to be 6.8 × 10−5, which is much lower than the [C6H−]/[C6H] ratio (0.093). From this result, the rate coefficient for the radiative attachment reaction between C4H and electron is estimated to be as small as 3 × 10−11 cm3s−1 on the basis of the simplified chemical model. The present observation has demonstrated the uniqueness and importance of L1527 in searching for a new carbon-chain molecule in a star-forming region.