Research

The details of my studies are  below. 

Nobeyama 45m survey of Galactic molecular clouds from low to high densities: Nobeyama-CIRCUS

Molecular clouds have long been known to exhibit long filamentary structures (e.g., Schneider & Elmegreen 1979). Herschel observations have confirmed that such filaments are truly ubiquitous in the cold interstellar medium (ISM) of the Milky Way (e.g., Schisano et al. 2020) and revealed an omnipresence of 0.1-pc width molecular filaments (e.g., Arzoumanian et al. 2011; Palmeirim et al. 2013; Cox et al. 2016; Arzoumanian et al. 2019).  Remarkably, ∼0.1pc-wide filaments are observed in both actively star-forming and quiescent, non-star-forming molecular clouds (cf. André et al. 2010; Miville-Deschênes et al. 2010). On the other hand, molecular line observations have reported the presence of variation in the filament width. The point is that the universality of the 0.1-pc width filament is still under debate. To reveal the universality of the filament width, the followings are required: i) observations toward filaments in the massive star-forming regions and extra galaxies, ii) observations with a high angular resolution to resolve the typical filament width of 0.1-pc, iii) continuum observations in the continuum to measure and compare the filament width under the same condition (i.e. observations in the same tracer).  However, there is a big issue to tackle it. There are no instruments capable of obtaining spatial resolution of much less than 0.1 pc in the continuum for filaments in massive star-forming regions at a few kpc.

To resolve this issue, we are trying to predict the H2 column density (i.e. continuum) map from the molecular lines by using the machine learning technique. The observations in several molecular lines to cover the wide range from low to high densities and toward several star-forming regions to correct the (abundance) variation among regions are required. 

In the 2020-2021 season, we conducted several massive star-forming regions in 12CO(1-0), 13CO(1-0), C18O(1-0), and CN(1-0) with Nobeyama 45m telescope.  In the 2021-2022 season, we conducted several massive star-forming regions in dense gas tracers such as H13CO+, HCO+, H13CN, and HCN  with Nobeyama 45m telescope. In the 2022-2023 season, we are planning to observe one low-mass star-forming region and one young massive star-forming region in multi-molecular lines with Nobeyama 45m telescope. 

Using the obtained observational data-set, we will also investigate the filament formation process and the process of the star formation in the filament from the kinematic view point.

This research is supported by KAKENHI 21H00057, 20K04035, and 19K23463.

Fig. Preliminary results of observations in the 2021-2022 season.

Member

Unbiased Survey for External Triggers of Star Formation 

To reveal the global perspective of star formation in GMCs and search for their external triggers, I conducted unbiased and extensive mapping observations of the Orion-A GMC in the 1.1-mm continuum emission with the AzTEC camera mounted on the ASTE and in the CO(1–0) emission with the 25 beam receiver on the 45-m telescope (Fig. 1) [1]. The 1.1-mm continuum and CO data provide us with information on the distribution of dense cores and the velocity structure of the cloud, respectively. With these data and those of MSX 8 μm and Spitzer 24 μm, I have investigated the detailed structure and kinematics of the dense gas associated with the Orion-A GMC and have discovered several pieces of evidence for external triggers for star formation. From the present survey, three types of external triggers for star formation have been revealed: 1) collision of the diffuse gas on the cloud surface, particularly at the eastern side of the OMC-2/3 region; 2) irradiation of UV radiation on pre-existing filaments and dense molecular cloud cores; and 3) molecular outflows. In addition, I conducted the wide-filed mapping in [CI], 13CO, and C18O lines. From these observations, I identified the region irradiated by the FUV radiation from the view points of the chemistry [2, 3]. The extensive wide-field and high-sensitivity imaging technique used by me was the first to reveal the ubiquitous external triggers of star formation in the Orion-A GMC. 

Fig 1. (a) AzTEC/ASTE 1.1mm-dust continuum map. The Ori-KL region is excessively bright for reconstructing an accurate structures with the AzTEC 1.1-mm data; therefore, the image of the Ori-KL region is an artifact in this figure. (b) CO peak intensity map.

Outflow Triggered Star Formation 

The observations in H13CO+ emission, which is known as a dense gas tracer, have revealed 0.07-pc-scale dense gas associated with FIR 4. A comparison of the distribution of the H13CO+ emission and the CO outflow from FIR 3 suggests an interaction between the dense gas associated with FIR 4 and the FIR 3 outflow. Furthermore, SiO and CH3OH emissions, which are known as shock tracers, have been detected near the interface between the outflow and dense gas. An increase in velocity width of the H13CO+, SiO, and the CH3OH lines is apparent at this interface from the ambient velocity width of 1 km s−1 up to 8 km s−1. These results indicate the presence shock resulting from the interaction between the outflow from FIR 3 and the dense gas associated with FIR 4. Conversely, NMA observations of FIR 4 in the 3.3-mm continuum emission have first revealed that FIR 4 consists of 11 dusty cores of mass ∼0.2–1.4 Msun The separation between these cores (∼5×103 AU) is on the same order of the Jeans length (∼13×103 AU), suggesting that the fragmentation into these cores has been caused by gravitational instability. The timescale of the fragmentation (∼3.8×104 yr) is similar to that of the interaction between the outflow and dense gas (∼1.4×104 yr). On the basis of these observational results, I have suggested that the interaction between the outflow from FIR 3 and the dense gas at FIR 4 has triggered the fragmentation into these dusty cores, which results in the next generation of star formation in FIR 4 [4]. In addition, I conducted the spectral-line survey toward FIR 4 at 82–106 GHz and 335–355 GHz and detected 120 molecular lines and 20 spices [5]. I found that the chemical compositions in FIR 4 are similar to those in another outflow-shocked region L1157 B1. Similar observational results supporting the outflow-triggered star formation have also been obtained in the OMC-2/FIR 6 region [6]. 

Fig 2. (left) Distribution of the CO(1-0) outflow from FIR 3 (red contours) and dense gas (H13CO+ emission) associated with FIR 4 (black contours). Green arrow show the direction of the FIR3 outflow. Open black circles show the field of view of the NMA observations. (rigth)3.3-mm dust-continuum map in the FIR 4 region observed with the Nobeyama Millimeter Array (NMA). Green plus signs, blue circles and red squares indicate positions of MIR sources, 3.6-cm free-free sources and the 1.3-mm dust-continuum emission [7].

Testing the universality of the star formation efficiency in dense gas 

Recent studies with, e.g., Spitzer and Herschel have suggested that star formation in dense molecular gas may be governed by essentially the same “law” in Galactic clouds and external galaxies. This conclusion remains controversial, however, in large part because different tracers have been used to probe the mass of dense molecular gas in Galactic and extragalactic studies, and because the HCN (1–0) and HCO+ (1–0) tracers often used in the extragalactic case rely on uncertain conversion factors. To calibrate the HCN line commonly used as dense gas tracers in extragalactic studies and to test the possible universality of the star formation efficiency in dense gas, SFEdense, I carried out wide-field mapping of the nearby clouds, using the same extragalactic tracer, HCN. These line intensities tend to be larger in stronger FUV field. I found that the empirical HCN conversion factor is significantly anti-correlated with the strength of the local FUV field. Re-estimating the dense gas masses in external galaxies with my G0-dependent conversion factor, I found that SFEdense is remarkably constant over 8 orders of magnitude in dense gas mass. These results suggest that the star formation efficiency in dense gas is constant over a wide range of 1-10 pc to >10 kpc, suggesting an universal “star formation law” converting into stars from the dense gas [9]. 

Fig. 3 SFE against Mdense.  The black solid line and black dashed lines display the simple empirical relation expected from the “microphysics" of prestellar core formation within filaments (see also André et al. 2014; Könyves et al. 2015):SFR=(4.5±2.5)×10−8M⊙yr−1 ×(Mdense/M⊙),i.e.,SFE=(4.5±2.5)×10−8yr−1. 

Engineering and Commissioning Observations 

I focused on testing and measurements of the 45m telescope, the NMA, and the ASTE. I significantly contributed to the establishment of the AzTEC/ASTE data reduction pipeline. An IDL-based data reduction pipeline developed by researchers at the University of Massachusetts encountered a difficulty in the removal of atmospheric components and reconstruction of the extended structure. A principal components analysis (PCA) cleaning method was adopted for the removal of atmospheric noise in this pipeline. However, this data reduction pipeline was optimized for point-like sources instead of extended source structures. Thus, I worked on an iterative mapping method [9], known as FRUIT, to recover the extended features. Through imaging simulations, I have verified that FRUIT can recover more than 80% of the total flux in my Orion map [1, 10]. 

Fig 4. (a) AzTEC PCA, (b) AzTEC FRUIT, and (c) SCUBA 850 um map. The same color scale is used for the three maps. The beam sizes of the AzTEC PCA, AzTEC FRUIT, and SCUBA maps are 30'', 40'', and 14'', respectively.

[References]

Shimajiri, Y., Kawabe, R., Takakuwa, S., et al. 2011, PASJ, 63, 105 

Shimajiri, Y., Sakai, T., Tsukagoshi, T., et al. 2013, PaJL, 774, L20 

Shimajiri, Y., Kitamura, Y., Saito, M., et al. 2014, A&A, 564, A68 

Shimajiri, Y., Takahashi, S., Takakuwa, S., Saito, M., & Kawabe, R. 2008, ApJ, 683, 255 

Shimajiri, Y., Sakai, T., Kitamura, Y., et al. 2015b, ApJS, 221, 31 

Shimajiri, Y., Takahashi, S., Takakuwa, S., Saito, M., & Kawabe, R. 2009, PASJ, 61, 1055 

Chini, R. et al. 1997, ApJL, 474, L135 

Shima jiri et al. 2017. 

Enoch, M. L., Young, K. E., Glenn, J., et al. 2006, ApJ, 638, 293 

Shimajiri et al. 2015a

Etc...