Keywords

9.1 Chemical Diversity

9.1.1 Chemical Diversity in a Disk Forming Region

As described in Chap. 1, it is well known that the chemical composition of the protostellar envelopes shows significant diversity at a 1000 au scale [20]. The two distinct cases are the warm carbon-chain chemistry (WCCC) and the hot corino chemistry. In this thesis, the chemical composition and its distribution in the vicinity of a protostar were investigated toward the five representative sources revealing the WCCC and the hot corino chemistry. it was confirmed that the chemical diversity recognized at a few 1000 au scale resolution by single-dish observations can also be seen at a 100 au scale by high angular resolution observations with ALMA (Table 9.1); WCCC sources, L1527 and IRAS 15398\(-\)3359 (Chaps. 4 and 5), are rich in unsaturated carbon-chain and related molecules (e.g. CCH and c-C\(_3\)H\(_2\)) at a few 100 au scale, while they are confirmed to be deficient in saturated complex organic molecules (COMs; e.g. HCOOCH\(_3\)) even with the high angular resolution and high sensitivity of ALMA. On the other hand, hot corino sources, IRAS 16293\(-\)2422 Source A and Source B (Chaps. 6 and 7), are confirmed to be rich in COMs, such as CH\(_3\)OH and HCOOCH\(_3\), and their distributions are highly concentrated in the vicinity of the protostar at a 100 au scale or smaller. These results suggest that the chemical diversity seen at a few 1000 au scale resolution is indeed seen in a more compact region at a 100 au scale for the above sources as it is.

Table 9.1 Chemical differentiation in low-mass protostellar sources
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On the other hand, L483 (Chap. 8), which was recognized as a WCCC candidate source based on single-dish observations [8, 9] shows a different feature. In this source, CCH is indeed abundant at a few 100 au scale with the ALMA observations, which is consistent with its WCCC characteristics. However, some COM lines (HNCO, NH\(_2\)CHO, and HCOOCH\(_3\)), which are specific to the hot corino chemistry, are also detected in the vicinity of the protostar at a 100 au scale or smaller. Since COMs are generally expected to be deficient in WCCC sources as shown in the above L1527 and IRAS 15398\(-\)3359 cases, the detection of the COM emission in L483 was completely unexpected. Hence, L483 shows the WCCC characteristics at the envelope scale (\(\sim \)a few 100 au), while it shows the hot corino activity at the disk formation scale (\(\sim \)100 au). Such a ‘hybrid’ character source was first identified definitively, although its existence was suggested by single-dish observations of carbon-chain molecules (CCH, C\(_4\)H, etc.) and CH\(_3\)OH [7, 18].

In this regard, [10] recently reported that the low-mass Class 0 source B335 reveals the hot corino chemistry. B335 is a representative Bok globule [13] and is regarded as one of the best test-bed source for the star formation studies (e.g. [4, 25]). They conducted the ALMA observation toward B335, and detected a number of COM lines (CH\(_3\)CHO, NH\(_2\)CHO, and HCOOCH\(_3\)) concentrated within a few 10 au around the protostar. Thus, B335 apparently harbors a hot corino. Nevertheless, they also found that this source shows extended distributions of CCH and c-C\(_3\)H\(_2\) over a scale of a few 100 au. Based on these results, B335 can be interpreted as a hybrid character source, as in the case of L483.

In any case, a 100 au scale is just a scale of protoplanetary disks. Thus, all the above results imply that the initial condition in chemical evolution from protoplanetary disks to planets already has diversity at the earliest evolutionary stages.

9.1.2 Which Kind of the Chemical Characteristics is Common?

WCCC and the hot corino chemistry are distinct cases of the chemical diversity of protostellar sources, and have often been thought to be exclusive to each other. The difference in the duration time of the starless core is suggested to be the cause of the difference of the chemical composition [18, 20]. In this picture, the chemical evolution of starless cores from the time when the external UV radiation is shielded to the time of the birth of a protostar is considered. In the initial stage of the chemical evolution, the carbon mostly exists in the ionized form (C\(^+\)) or the atomic form (C), and they are gradually converted to CO through gas-phase chemical reactions. The time scale of the conversion (\(\tau _\textrm{chem}\)) is estimated to be a few \(10^5\) yr. The time scale is determined by the H\(_3^+\) density, which is roughly independent of the H\(_2\) density [22, 24].Footnote 1 The chemical time scale is comparable to the dynamical time scale; the free fall time is represented as follows:

$$\begin{aligned} \tau _\textrm{ff}&= \sqrt{\frac{3 \pi }{32 G \rho _0}}\end{aligned}$$
(9.1)
$$\begin{aligned}&\propto n (\textrm{H}_2)^{-\frac{1}{2}}, \end{aligned}$$
(9.2)

where G denotes the gravitational constant, and \(\rho _0\) the initial mass density. The time scale of the molecular adsorption onto the dust surface (the depletion time scale) is roughly estimated to be as follows e.g. [24]:

$$\begin{aligned} \tau _\textrm{d}&= \frac{10^9}{n (\textrm{H}_2)} \textrm{yr} \end{aligned}$$
(9.3)
$$\begin{aligned}&\propto n (\textrm{H}_2)^{-1}. \end{aligned}$$
(9.4)

Figure 9.1 shows the relation between the above time scales and the H\(_2\) density. These time scales (\(\sim 10^5\) yr) are comparable to that of the starless-core stage (a few \(10^5\) yr; Chap. 1).

Fig. 9.1
figure 1

Various time scales as functions of H\(_2\) density. If the depletion time scale is longer than the time scale of the chemical equilibrium, the atomic C is converted to CO, and then CO is adsorbed onto the dust grains. If the depletion time scale is shorter than the time scale of the chemical equilibrium, the atomic C is adsorbed onto the dust grains as it is. This mechanism causes the diversity of the chemical composition of grain mantle, and thereby that of protostellar cores

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If the duration time of such a starless core phase is long, the core stays in a moderately dense condition for a long time. Then the gas-phase carbon is mostly converted to CO, and then CO is depleted onto dust grains, as the density increases (Fig. 9.2a). The CO molecules on dust grains are subject to hydrogenation reactions with the H atom to form CH\(_3\)OH and probably COMs such as CH\(_3\)CHO, HCOOCH\(_3\), and (CH\(_3\))\(_2\)O. After the onset of star formation, dust grains are heated up, and COMs are evaporated. This is the hot corino chemistry. If the duration time is, in contrast, short, the C atom is mainly depleted onto dust grains before it is converted to CO in the gas phase (Fig. 9.2b). The C atoms on dust grains are hydrogenated to form CH\(_4\). After the onset of star formation, CH\(_4\) comes out of dust grains, and produces various carbon-chain molecules in the gas phase as:

$$\begin{aligned} \textrm{C}^+ + \textrm{CH}_4&\rightarrow \textrm{C}_2 \textrm{H}_3^+ + \textrm{H}, \end{aligned}$$
(9.5)
$$\begin{aligned} \textrm{C}_2 \textrm{H}_3^+ + \textrm{e}^-&\rightarrow \textrm{C}_2 \textrm{H}_2 + \textrm{H}, \textrm{C}_2 \textrm{H} + \textrm{H} + \textrm{H}. \end{aligned}$$
(9.6)

This is the WCCC. The basic picture of these processes are confirmed by the chemical model calculations [1, 6]. The exclusive nature of WCCC and hot corino chemistry can thus be explained. At the same time, the existence of a source with a hybrid (or intermediate) chemical characteristics is also expected in this scheme, because the depletion of the mixture of C and CO can occur for an appropriate duration time.

Fig. 9.2
figure 2

Taken from [18]. © AAS. Reproduced with permission

Schematic illustration of the depletion of CO and atomic C onto dust grains and their conversion into COMs and CH\(_4\). (a) Long and (b) short duration times favor the hot corino chemistry and WCCC, respectively.

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Both the WCCC and the hot corino chemistry are triggered by evaporation of molecular species from dust grains. However, the temperature necessary for these two processes are different. The WCCC is triggered by evaporation of CH\(_4\), which occurs in a warm region above 25–30 K. On the other hand, the hot corino chemistry is triggered by evaporation of COMs or disruption of ice mantle of dust grains, which occurs in a hot region above 100 K. Hence, the WCCC region is generally larger than the hot corino. Before the ALMA era, the both regions are sufficiently smaller than the observation beams, and hence, they are just regarded as the chemical feature of protostellar cores. However, these two regions are now resolved at a high angular resolution with ALMA. In fact, the above L483 case seems to be a good example of a source with the hybrid characteristics.

In fact, the hybrid chemical characteristics is consistent with the chemical model calculations. Figure 9.3 shows the result of the chemical model calculation for a contracting protostellar core [1]. In this model, CH\(_4\) is evaporated at the radius of 1000 au from the protostar, and triggers the WCCC. Indeed, the abundances of carbon-chain molecules start to be enhanced inward of the evaporation radius of CH\(_4\). On the other hand, COMs appears in the inner region (<100 au), whose temperature is higher than 100 K. Hence, the hybrid chemical characteristics, which is identified in L483 with ALMA, may represent a ‘standard’ case of the chemical composition of protostellar cores. The recent result for B335 also supports this idea. So far, the representative WCCC sources, L1527 and IRAS 15398\(-\)3359, and the representative hot corino source, IRAS 16293\(-\)2422, have extensively been studied because of their peculiar chemical nature. However, they should be regarded as the extreme cases, and the hybrid (or intermediate) character sources, such as L483 and B335, would be more common. In order to assess this possibility, statistical studies are required. Such studies will also provide us an important clue to understanding the ‘environmental factor’ responsible for the appearance of the distinct cases.

Fig. 9.3
figure 3

Taken from [1] with some modifications. © AAS. Reproduced with permission

Radial distribution of a the gas temperature and b−d those of molecules in the protostellar core at the final stage of the chemical model of the contracting cloud. Black lines represent ice-mantle species, while gray lines represent gas-phase species.

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9.2 Chemical Change

9.2.1 Drastic Chemical Change Around the Centrifugal Barrier

In this thesis, chemical changes across the centrifugal barrier have been found in all the observed protostellar sources. As shown in Fig. 9.4, different molecules trace different physical components around the protostar.

In general, the chemical composition of the gas is changed by gas-phase reactions and/or gas-grain interactions (i.e., evaporation of molecules from dust grains as well as depletion of molecules onto dust grains). For instance, CCH is thought to be broken up by the gas-phase reaction, such as:

$$\begin{aligned} \textrm{CCH} + \textrm{O}&\rightarrow \textrm{CH} + \textrm{CO}, \end{aligned}$$
(9.7)
$$\begin{aligned} \textrm{He}^+ + \textrm{CCH}&\rightarrow \textrm{C}^+ + \textrm{CH} + \textrm{He}, \end{aligned}$$
(9.8)
$$\begin{aligned} \textrm{H}^+ + \textrm{CCH}&\rightarrow \textrm{C}_2^+ + \textrm{H}_2. \end{aligned}$$
(9.9)

Alternatively, CCH would be depleted onto dust grains in the mid-plane just inside the centrifugal barrier, if the dust temperature is lower than its desorption temperature of about 40 K. The time scale of the depletion of molecules onto dust grains is given as: \(t_\textrm{dep} \sim 10^9 / n (\textrm{H}_2)\) yr. Since the mid-plane density just inside the centrifugal barrier is \(10^7-10^8\) cm\(^{-3}\) and the temperature is about 30 K for L1527 [19], the depletion time scale is shorter than the rotation period at the centrifugal barrier (\(\sim \)10\(^3\) yr). Thus, CCH is lost in the gas-phase while it goes around the protostar at the centrifugal barrier, and it would not be delivered into the gas phase of the disk component inside the centrifugal barrier. In fact, CCH traces the envelope component, but not the disk component in WCCC sources in Table 9.1. Note that CCH may be delivered to the disk component in the solid phase. However, it will readily be polymerized, and never appears in the gas phase.

Fig. 9.4
figure 4

Schematic illustration of the major tracers of the envelope component, the centrifugal barrier (‘CB’), and the disk component in hot corino sources (left), WCCC sources (right), and the hybrid case of them (center)

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A similar situation may occur for CS. CS is generally abundant in the gas phase of dense molecular clouds e.g. [23], and hence, they also exist in the infalling-rotating envelope. If the mid-plane temperature is lower than the desorption temperature of CS (35 K; See Appendix A), the gas-phase CS molecules falling from the envelope will be depleted onto dust grains while they are rotating (or stagnated) around the centrifugal barrier. CS is also destroyed by the ionic species like He\(^+\) and H\(^+\). Note that, in contrast to the CCH case, the CS \(+\) O reaction has the activation energy of 783 K, so that it is only effective at higher temperature. As a result, CS mainly traces the infalling-rotating envelope in L1527. If the mid-plane temperature is higher than the desorption temperature of CS, CS is not frozen out onto dust grains and can survive in the gas phase. This situation seems to be seen in L483, because the luminosity of L483 (13 \(L_\odot \) [3]) is higher than that of L1527 (1.7 \(L_\odot \) [5]), as discussed in Chap. 8.

The chemical composition can also be changed by evaporation of molecules from dust grains. There are two mechanisms that liberate surface molecules into the gas phase; those are shock heating and protostellar heating. In the infalling-rotating envelope, the gas is stagnated in front of the centrifugal barrier, and the accreting gas with the infall velocity of a few km s\(^{-1}\) causes a weak accretion shock [2, 15]. Then the temperature is raised near the shock front, which evaporates the surface molecules. The SO molecules in L1527 seem to be evaporated from dust grains in this mechanism in front of the centrifugal barrier. Indeed, the temperature is found to be raised up to 60 K or higher around the centrifugal barrier [15, 17]. In IRAS 16293\(-\)2422 Source A, a similar enhancement of the temperature around the centrifugal barrier is observed, as described in Chap. 6. This will cause the evaporation of COMs there, which would be the origin of the hot corino chemistry.

Another mechanism responsible for desorption of molecules is the protostellar heating. Generally, the dust temperature becomes higher as approaching to the protostar due to the illumination by the protostar [11, 14]. Since the desorption temperature is different from molecule to molecule, as described in Appendix A, one may expect that this causes chemical differentiation as a function of the radius from the protostar. However, the actual distribution of molecules is not as simple as expected. The different distributions of OCS and H\(_2\)CS in IRAS 16293\(-\)2422 Source A, which have similar desorption temperature, are not explained in this way. On the other hand, the contribution of this mechanism seems important for CS in L483, as mentioned above. In addition, the protostellar heating in combination with the geometrical effect of the centrifugal barrier may also play an important role of the distribution of COMs in IRAS 16293\(-\)2422 Source A, as discussed in Chap. 6. Although the contribution of the protostellar heating to desorption of molecules cannot be ruled out, the physical changes around the centrifugal barrier plays an important role in the chemical differentiation.

9.2.2 Tracers in WCCC and Hot Corino Sources

As demonstrated in Chaps. 4–8, the chemical compositions are useful to disentangle the physical components in the disk-forming regions. For such chemical diagnostics, it is essential to understand what kind of molecular species trace which part of the disk/envelope system.

As shown in Table 9.1 and Fig. 9.4, the chemical changes are seen in different molecules in different sources. The envelope components are traced by unsaturated organic molecules (i.e. carbon-chain and related molecules) in the WCCC sources (L1527, IRAS 15398\(-\)3359, and TMC−1A), while it is traced by OCS and H\(_2\)CS in the hot corino sources (IRAS 16293\(-\)2422 Source A and Source B). The centrifugal barriers are highlighted by SO in the WCCC sources and by COMs in hot corino sources. The disk components inside the centrifugal barrier is traced by various molecules, such as H\(_2\)CO, H\(_2\)CS, SO, and COMs, depending on sources. It should be noted that the emitting region of one molecule is also different from source to source, as shown in the CS case.

Based on the above results, useful tracers of the disk/envelope system seem to depend on the chemical characteristics of sources, although some trends can be recognized. Thus, confirmation of the applicabilities of the molecules listed in Table 9.1 as the tracers of the physical components in various sources is essential to the establishment of chemical diagnostics for the disk-forming regions. Moreover, characterizing the chemical behavior of more molecular species is also awaited.