L1448-MM OBSERVATIONS BY THE HERSCHEL KEY PROGRAM, "DUST, ICE, AND GAS IN TIME" (DIGIT)

PACS is a 5×5 array of 94 × 94 spatial pixels (hereafter referred to as spaxels) covering the spectral range from 50–210 μm with λ/δλ ∼ 1000–3000, divided into four segments, covering λ ∼ 50–75, 70–105, 100–145, and 140–210 μm. L1448-MM was observed on 2011 February 2 (λ ∼ 50–75 and 100–145 μm; AOR: 1342213683) and 2011 February 22 (λ ∼ 70–100 and 140–210 μm; AOR: 1342214675) in the range scan mode of PACS with a single footprint.

The telescope and sky background emission was subtracted using two nod positions 6' from the source in opposite directions. Each segment was reduced using the "calibration block" pipeline from HIPE v8.1 (Ott 2010), and flux calibrated to an extraction from HIPE v6.1. The former extraction produces spectra of higher signal-to-noise ratio (S/N), while the latter provides better absolute flux calibration. This process and the reasoning behind using two different HIPE versions are described in detail in Green et al. (2013). Green et al. (2013) applied consistent methods to all sources to correct for the extended emission in continuum and line. However, the methods are not optimized for targets with multiple sources. In the case of L1448-MM, we observed clear evidence of multiple emitting sources within the PACS field of view (FOV). As a result, we used modified methods to extract exact continuum/line fluxes, so our fluxes are slightly different from those in Green et al. (2013). However, the main concept for our flux measurements is similar to Green et al. (2013); only the HIPE v6.1 reduction was used for the spectral energy distribution (SED) extraction, but the combination of the HIPE v6.1 and HIPE v8.1 reductions was adopted for line fluxes.

Unlike isolated sources, it is hard to correct for the point-spread function (PSF) in L1448-MM because it has multiple sources. As a result, we added the whole fluxes over 25 spaxels to present the total continuum flux. (The decomposition of the fluxes by the multiple sources is explained in Section 3.2.) For the line fluxes, we calculated the equivalent width (EW) of each emission line from the HIPE v8.1 reduction, then multiplied the EW by the local continuum determined from the HIPE v6.1 reduction to calculate total line fluxes over the whole 25 spaxels, fitting a first-order polynomial baseline to local continuum. Line widths and continuum levels were fitted with the Spectroscopic Modeling Analysis and Reduction Tool (Higdon et al. 2004), which was originally developed for data analysis of the Infrared Spectrograph (IRS; Houck et al. 2004) on Spitzer.

For the total line fluxes over the whole L1448-MM region covered by PACS, we used the EWs of CO, H₂O, and OH lines measured in the spectra extracted from the two spaxels, where the lines are the strongest and two point sources, L1448-MM(A) and L1448-MM(B) are located. We compared the EWs measured from these two spaxels with the EWs measured from a sum over all 25 spaxels to determine if there was any sign of spatially extended CO, H₂O, or OH lines. In this analysis we found that the EWs were not different in the two cases, and the line fluxes increased as the PSF would predict. However, for [O i] lines, the EW measured from all spaxels is much greater than that measured from the two spaxels, even after accounting for broadening due to the PSF. [O i] lines are detectable over four spaxels (see Figure 15), and the EWs from the four spaxels are consistent with the EWs from the whole 25 spaxels although the four spaxel EWs have lower measurement errors. As a result, we adopted the four spaxel EWs for [O i] lines to calculate total [O i] line fluxes. The procedure of measuring line fluxes as well as the error calculation is described in the Appendix of this work. According to the measured EWs, all observed lines are spectrally unresolved in the PACS observations. As mentioned above, L1448-MM is composed of multiple sources; therefore, we also measured fluxes of each source separately. The simple method we used to decompose the multiple sources into separate fluxes is described in Section 3.2.

The area around L1448-MM was mapped with Spitzer/IRS in 2008 February (Neufeld et al. 2009; Giannini et al. 2011). H₂ rotational lines (S(0)–S(7)) in the range of 5 ∼ 38 μm were detected. We convolved these H₂ maps with the PACS spaxels to compare with our PACS maps. Refer to Neufeld et al. (2009) and Giannini et al. (2011) for the details of these observations.

Additionally, we present observations of the CO J = 2–1 transition at 230.537970 GHz, mapped with the 6 m telescope at Seoul Radio Astronomy Observatory (hereafter SRAO) in 2010 May. The beam FWHM is 48'' at 230 GHz and the velocity resolution is 0.127 km s⁻¹ after binning by two channels. The main beam efficiency and pointing accuracy are 0.57 and ∼3'', respectively.

Finally we include Spitzer observations of L1448-MM. The IRAC and MIPS images used here have been downloaded from the Spitzer data archive. However, we adopt the point source fluxes at the IRAC and MIPS bands from Jørgensen et al. (2006). The IRS Short–High and Long–High data obtained by Nisini in (2006) have been also downloaded from the Spitzer data archive and reduced with the same method used in Furlan et al. (2006).

In the IRAC images, the trail of sources aligns in the SE–NW direction (Figure 1), which is consistent with the large-scale outflows. There is an additional point-like source between L1448-MM(A) and MM(B) in the IRAC band 3 and 4 images, which has been reported as a tertiary 1.3 mm continuum source (Maury et al. 2010). Owing to decreased resolution in MIPS band 2, L1448-MM(A) and MM(B) are not resolved in that image, although they seem distinct from each other in images at shorter wavelength. Hereafter, we designate L1448-MM(A) and L1448-MM(B) as MM(A) and MM(B), respectively.

Zoom In Zoom Out Reset image size

Here we present the SEDs of two point sources, MM(A) and MM(B) collected for wavelengths ranging from a few microns to submillimeter. In Figure 2, the IR data points come from Jørgensen et al. (2006) and fluxes at millimeter wavelengths are taken from Curiel et al. (1990), Jørgensen et al. (2007), Maury et al. (2010), and Hirano et al. (2010). The IRS spectra of MM(A) and MM(B) are also plotted in Figure 2.

Zoom In Zoom Out Reset image size

In the PACS range (Figure 2), the black line represents the spectrum extracted from all 25 spaxels (see Section 2.1) while the blue and red lines show the spectra of MM(A) and MM(B), separately. All three spectra (black, blue, and red) have been extracted from the HIPE v6.1 reduction, which provides superior flux calibration. In the PACS footprint, MM(A) and MM(B) are located near the central spaxel and the spaxel in the right south of the central spaxel, which are designated as spaxel C and S, respectively, as seen in Figure 3. The coordinates of spaxel C are the same as the coordinates of MM(A), but the coordinates of spaxel S, (3^h25^m390, +30°43'568), are slightly different from those of MM(B).

Zoom In Zoom Out Reset image size

To obtain the spectra of MM(A) and MM(B), we (1) extracted spectra from the spaxels, C and S, respectively, (2) scaled to match continuum levels around 100 μm, and (3) corrected for the effect of the PSF of PACS to each source.

Since the spectra of each segment (λ ∼ 50–75, 70–105, 100–145, 140–210 μm) are not matched smoothly, we scaled up or down SEDs of each segment to match continuum levels around 100 μm. The scale factors applied to the spectrum extracted from spaxel C are 1.052, 0.918, and 0.854 in the range of λ < 72, 101–142, and >142 μm, respectively. The scale factors for the spectrum extracted from spaxel S are 0.903, 0.667, and 0.727, in the same wavelength range.

In order to correct for the PSF, that is, to decompose fluxes of MM(A) and MM(B), we assume that only two point-like sources contribute the emission in the two spaxels and calculate the contribution of each source to the normalized fluxes of two spaxels using the PSF.⁷ The contribution of a point source to the spaxel where it is located is 80% at 50 μm and 32% at 200 μm, and to the adjacent spaxel is ∼3.5% at 50 μm and ∼10% at 200 μm. As a result, at a given wavelength, the fluxes of the two point sources are decomposed by solving two simple linear simultaneous equations as below:

$$\begin{equation} F_{\lambda ,\rm C} = P_{\lambda ,\rm AC} * X_{\lambda ,\rm A} + P_{\lambda ,\rm BC} * X_{\lambda ,\rm B} \end{equation} \tag{ 1 }$$

$$\begin{equation} F_{\lambda ,\rm S} = P_{\lambda ,\rm AS} * X_{\lambda ,\rm A} + P_{\lambda ,\rm BS} * X_{\lambda ,\rm B}, \end{equation} \tag{ 2 }$$

F_{λ, C} and F_{λ, S} are fluxes measured from the spectra extracted at spaxel C and S, while X_{λ, A} and X_{λ, B} are the decomposed fluxes of MM(A) and MM(B), respectively. P_{λ, AC} and P_{λ, BC} are the contribution to spaxel C by MM(A) and MM(B), while P_{λ, AS} and P_{λ, BS} are the contribution to spaxel S by MM(A) and MM(B), respectively. P_{λ, AC}, P_{λ, BC}, P_{λ, AS}, and P_{λ, BS} were calculated with the exact coordinates of MM(A) and MM(B).

The slope of the SEDs (rising into the FIR) demonstrates that cold material is present showing that both MM(A) and MM(B) are deeply embedded. The continuum luminosity integrated in the PACS wavelength range is about 3.5 and 0.9 L_☉ for MM(A) and MM(B), respectively, as listed in Table 4. The bolometric luminosities of MM(A) and MM(B), which are calculated with their decomposed SEDs, are ∼5.5 and 1.7 L_☉ (see Section 5).

In the IRS range, the flux of MM(B) is flatter than that of MM(A) longward of 15 μm. In the IRS spectrum of MM(B), the flux drops shortward of 15 μm. The excess flux in the IRAC bands compared to what is extrapolated from the IRS SED might be attributed to some shocked gas and scattered light around MM(B).

A deep CO₂ 15.2 μm ice absorption feature has been detected in both MM(A) and MM(B) (Figure 4), indicative of the existence of dense envelopes in both sources. Both CO₂ absorption profiles show the broad red wing, which is attributed to the CO₂ and H₂O ice mixture. Although the S/N is somewhat low, we see clearly a double-peaked feature, indicative of the pure CO₂ ice component, which exists only toward protostars (Pontoppidan et al. 2008). In the millimeter wavelength range (700 <λ < 3000 μm), the spectral index (−dlogS_ν/dlogν, where S_ν is the flux density, and ν is the frequency) is about 2.4 for MM(A) and 2.2 for MM(B). These large (>2) positive values imply that the millimeter continuum emission comes from dust, not from ionized gas. These SEDs and the CO₂ ice absorption feature, including the pure CO₂ ice feature, indicate that both sources are deeply embedded YSOs. The available millimeter data, however, do not constrain the dust properties further.

Zoom In Zoom Out Reset image size

The PACS spectra show numerous molecular lines of CO, H₂O, and OH in addition to atomic [O i] lines (Figure 5). The detected lines are listed in Table 1 including the upper level energy, E_u = (E_upper/k) (k is Boltzmann constant), and the Einstein coefficient, A_ul. The detected lines of each species are presented in Figures 6–9, where we present lines extracted only from the central spaxel. As the central spaxel contains most of the continuum, many lines were detected only at the central spaxel. Dionatos et al. (2009) calculated a visual extinction (A_V) of 11 and 32 mag toward MM(A) and MM(B), respectively, using the 9.7 μm silicate absorption feature of the IRS spectra. These A_V values attenuate line fluxes by less than 10% at λ ⩾ 60 μm. Therefore, we did not correct for the reddening in line fluxes.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Table 1. Detected Lines in the Herschel/PACS Spectrum of L1448-MM

Species	Transition	Eu	Aul	λ	Flux (10−18 W m−2)
		(K)	(cm−1)	(μm)	5 ×5a	(A)b	(B)c
CO	40–39	4512.67	0.00461300	65.69	67 ± 26	77	...
	39–38	4293.64	0.00436500	67.34	60 ± 19	55	...
	38–37	4079.98	0.00412000	69.07	74 ± 22	67	...
	37–36	3871.69	0.00387800	70.91	61 ± 25	58	...
	36–35	3668.78	0.00363800	72.84	104 ± 39	104	...
	35–34	3471.27	0.00340400	74.89	91 ± 27	166d	...
	34–33	3279.15	0.00317500	77.06	123 ± 36	88	...
	33–32	3092.45	0.00295200	79.36	83 ± 27	...	...
	32–31	2911.15	0.00273500	81.81	121 ± 36	110	12
	30–29	2564.83	0.00232100	87.19	172 ± 54	117	45
	29–28	2399.82	0.00212600	90.16	214 ± 64	139	40
	28–27	2240.24	0.00194000	93.35	214 ± 65	167	38
	27–26	2086.12	0.00176100	96.77	229 ± 75	182	...
	25–24	1794.23	0.00143200	104.44	195 ± 56	152	59
	24–23	1656.47	0.00128100	108.76	234 ± 68	203	65
	22–21	1397.38	0.00100600	118.58	288 ± 81	217	77
	21–20	1276.05	0.000883300	124.19	328 ± 92	236	90
	20–19	1160.20	0.000769500	130.37	350 ± 99	256	100
	19–18	1049.84	0.000665000	137.20	381 ± 108	259	128
	18–17	944.970	0.000569500	144.78	422 ± 120	294	122
	17–16	845.590	0.000482900	153.27	433 ± 123	300	146
	16–15	751.720	0.000405000	162.81	503 ± 142	318	206
	15–14	663.350	0.000335400	173.63	547 ± 155	373	222
	14–13	580.490	0.000273900	186.00	507 ± 144	303	211
OH	$\frac{1}{2}$, $\frac{9}{2}$–$\frac{1}{2}$, $\frac{7}{2}$	875.100	2.18200	55.89	87 ± 32	70e	...
		875.100	2.17500	55.95	88 ± 33	70e	...
	$\frac{3}{2}$, $\frac{9}{2}$–$\frac{3}{2}$, $\frac{7}{2}$	512.100	1.27600	65.13	312 ± 96	332	...
		510.900	1.26700	65.28	141 ± 41	129	...
	$\frac{1}{2}$, $\frac{7}{2}$–$\frac{1}{2}$, $\frac{5}{2}$	617.600	1.01400	71.17	100 ± 29	76	13
		617.900	1.01200	71.22	101 ± 29	76	13
	$\frac{1}{2}$, $\frac{1}{2}$–$\frac{3}{2}$, $\frac{3}{2}$	181.900	0.0360600	79.12	190 ± 59	172	26
		181.700	0.0359800	79.18	167 ± 49	154	39
	$\frac{3}{2}$, $\frac{7}{2}$–$\frac{3}{2}$, $\frac{5}{2}$	290.500	0.520200	84.60	251 ± 73	207	29
	$\frac{1}{2}$, $\frac{3}{2}$–$\frac{3}{2}$, $\frac{5}{2}$	270.200	0.00927000	96.31	37 ± 21	...	...
		269.800	0.00925000	96.37	15 ± 17	...	...
	$\frac{3}{2}$, $\frac{5}{2}$–$\frac{3}{2}$, $\frac{3}{2}$	120.700	0.138800	119.23	202 ± 57	124	77
		120.500	0.138000	119.44	249 ± 73	145	102
	$\frac{1}{2}$, $\frac{3}{2}$–$\frac{1}{2}$, $\frac{1}{2}$	270.200	0.0648300	163.12	53 ± 16	46	7
		269.800	0.0645000	163.40	58 ± 17	49	2
p-H2O	431–322	552.300	1.45200	56.33	89 ± 42	81	...
	919–808	1324.00	2.48600	56.77	125 ± 39	...f	...
	422–313	454.300	0.378500	57.64	120 ± 37	151	...
	726–615	1021.00	1.33800	59.99	69 ± 24	...f	...
	808–717	1070.60	1.74200	63.46	61 ± 23	84	...
	331–220	410.400	1.22200	67.09	137 ± 41	94	24
	524–413	598.800	0.667900	71.07	131 ± 40	123	...
	717–606	843.800	1.17800	71.54	96 ± 28	88	...
	615–524	781.100	0.452600	78.93	96 ± 32	108	...
	606–515	642.700	0.713200	83.28	165 ± 48	156	7
	322–211	296.800	0.352400	89.99	298 ± 95	233	39
	515–404	469.900	0.446000	95.63	276 ± 78	238	43
	220–111	195.900	0.260700	100.98	342 ± 117	...f	...
	404–313	319.500	0.172700	125.35	285 ± 81	209	54
	331–322	410.400	0.0784800	126.71	24 ± 8	...	...
	313–202	204.700	0.125100	138.53	520 ± 147	371	181
	413–322	396.400	0.0331600	144.52	124 ± 38	127	9
	322–313	296.800	0.0524600	156.19	239 ± 70	202	38
	413–404	396.400	0.0372600	187.11	32 ± 13	...	...
o-H2O	432–321	550.400	1.37400	58.70	184 ± 53	177	39
	818–707	1070.70	1.75100	63.32	210 ± 67	199	...
	716–625	1013.20	0.950800	66.09	125 ± 43	139	...
	330–221	410.700	1.24300	66.44	259 ± 76	221	41
	330–303	410.700	0.00850500	67.27	127 ± 37	124	18
	707–616	843.500	1.15700	71.95	276 ± 80	257	24
	321–212	305.300	0.331800	75.38	617 ± 176	416	152
	423–312	432.200	0.483800	78.74	522 ± 147	424	82
	616–505	643.500	0.749100	82.03	417 ± 118	359	45
	625–616	795.500	0.174000	94.64	113 ± 32	65	...
	441–432	702.300	0.152800	94.71	68 ± 19	57	...
	505–414	468.100	0.390200	99.49	547 ± 170	366	99
	514–423	574.700	0.156600	100.91	247 ± 90	192	...
	221–110	194.100	0.256400	108.07	735 ± 210	419	296
	432–423	550.400	0.122900	121.72	52 ± 15	52	13
	423–414	432.200	0.0808400	132.41	173 ± 50	123	33
	514–505	574.700	0.0757900	134.93	70 ± 22	50	13
	330–321	410.700	0.0661900	136.50	117 ± 34	99	13
	532–523	732.100	0.0817400	160.51	48 ± 14	...	...
	303–212	196.800	0.0504800	174.63	824 ± 233	510	355
	212–101	114.400	0.0559300	179.53	1173 ± 334	615	579
	221–212	194.100	0.0305800	180.49	337 ± 96	184	149
[O i]	3P1–3P2	227.712	0.0000891	63.18	922g ± 275	383	230
[O i]	3P0–3P1	326.579	0.0000175	145.53	84h ± 30	18	26

Notes. Emissions of CO and OH (84 μm) and CO and o-H₂O (113 μm) are detected, but not measured since they are blended. CO J = 13–12 is detected but not included in our analysis because of low responsivity of PACS in this wavelength range. ^aFrom the whole 5 × 5 spaxel. ^bFrom the unresolved gas component located at the position of MM(A). ^cFrom the unresolved gas component located at the position of MM(B). ^dThe line in the spectrum extracted from the central spaxel seems contaminated by an unknown feature, which possibly increases the EW of the line. ^eThis rotation transition was not split in the spectrum extracted from the central spaxel while it was split in the spectrum extracted over 25 spaxels. ^fWe excluded the lines because the line shapes are too strange to be fitted by a Gaussian profile. ^gEW measured from four spaxels. ^hEW measured from three spaxels.

Download table as:  ASCIITypeset images: 1 2

CO transitions were detected from J = 13–12 up to J = 40–39. The highest OH transition is ²Π_1/2 J = 9/2–7/2. Both lines of [O i] at 63 μm and 145 μm and 22 ortho- and 19 para-H₂O lines were detected. No H$_2^{18}$O or ¹³CO lines were detected. The upper limits of H$_2^{18}$O (2₁₂–1₀₁) and ¹³CO (15–14) are 1.70 × 10⁻¹⁷ and 1.73 × 10⁻¹⁷ W m⁻², respectively. Assuming $[^{16}\rm O]/[^{18}\rm O] = 550$ and $[^{12}\rm C]/[^{13}\rm C]=90$, the upper limits of optical depth of ¹²CO (15–14) and H$_2^{16}$O (2₁₂–1₀₁) are about 3 and 8, respectively, in the assumption of equal excitation temperatures for the isotopologues.

Figure 3 compares the IR image in the Spitzer IRAC band 2 with the SRAO CO J = 2–1 outflow map. The footprint of PACS is marked on the image. The IRAC 2 image shows a jet-like structure along a NW and SE path. The SRAO CO J = 2–1 outflow falls along this axis. The jet-like structure is more prominent to the NW in the IRAC 2 image. Nisini et al. (2000) noted that the adjacent area of the southern lobe has a higher visual extinction than that of northern lobe.

Figure 10 shows the distribution of continuum emission at 67 μm and 190 μm, extracted from the PACS data cube, compared to the MIPS 1 image. The continuum peaks close to MM(A) and slightly extends to MM(B), which is probably due to the contribution of MM(B) in the dust continuum. These two sources were decomposed into the continuum emission as described in Section 3.2.

Zoom In Zoom Out Reset image size

From Figure 11 to Figure 15, the maps of selected lines are presented. In each figure, a contour map is overlaid on the spectral map. Two transitions are selected for each species to study the spatial variation in the emission lines depending on energy level or wavelength; lines representing higher energy levels concentrate on spaxel C while lines of lower energy levels peak 4''–5'' toward the south. In addition, the low energy lines tend to distribute more broadly compared to lines from high energy levels. CO (J = 14–13, 186 μm), OH (²Π_3/2 J = 5/2–3/2, 119 μm), o-H₂O ($J_{K_{-1},K_1}$ = 2₁₂–1₀₁, 179 μm), p-H₂O ($J_{K_{-1},K_1}$ = 3₁₃–2₀₂, 138 μm), and [O i] (³P₁ − ³P₂, 63 μm) are the most evident examples of the spatially broadened distribution. The broad distribution of line emission at longer wavelengths could be real or the effect of the larger PSF at longer wavelengths. Nevertheless, the distribution of [O i] emission appears to be significantly different from the other species as the line flux of the southernmost spaxel is as strong as that at the spaxel S, indicating a relatively flat emission profile over several spaxels.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The line emission does not necessarily correspond to continuum sources spatially. However, the line emission (except for [O i] lines) is the strongest at the spaxels C and S as is the continuum emission. In addition, the peaks of the contour maps both in continuum and line emission are shifted to the south of MM(A) similarly. Therefore, the line emission might originate from two unresolved gas components located at spaxels C and S. In order to test this idea, we decomposed the line fluxes with the same method as used in the decomposition of the continuum (Section 3.2) by assuming that two unresolved gas components are located at the same positions of MM(A) and MM(B), which is not necessarily true. The decoupled line fluxes are listed in Table 1. Interestingly, the sum of the decomposed molecular line fluxes is consistent with the total fluxes over 25 spaxels within errors while the total [O i] 63 μm flux over 25 spaxels is greater than the sum of the decomposed fluxes by a factor of ∼2. Therefore, for CO, OH, and H₂O, the extended emission beyond these two spaxels is mostly caused by the PSFs of the two unresolved gas components while [O i] emission is indeed extended. Figure 15 also shows the elongated emission along the outflow direction, which indicates that the extended [O i] emission is connected to the outflow.

Figure 16 presents IRS line maps of the rotational transitions of H₂ compared with PACS emission lines and the IRAC 4.5 μm image. The H₂ emission is not spatially coincident with the FIR emission. H₂ emission is detected primarily from the blueshifted outflow to the north, where n(H₂) is higher and A_V is lower (Nisini et al. 2000; Dionatos et al. 2009), while the extended [O i] emission seems to originate from the position of the redshifted outflow to the south. Therefore, this overall feature of the mid-IR (MIR) and FIR emission suggests that the outflow source is located rather close to the surface of the associated molecular cloud. As a result, the column density of shocked material in the blue component is not large enough to block the MIR emission while the column density of shocked material in the red component is large enough to obscure the MIR emission and to produce the strong FIR emission line.

Zoom In Zoom Out Reset image size

This simple excitation analysis assumes that the lines are optically thin. If the populations can be fitted with a single line, the level populations can be characterized by a single temperature (T_rot). In the case of LTE, T_rot is the same as the kinetic temperature of the gas. A detailed description for this rotation diagram can be found in Green et al. (2013) as well as Goldsmith & Langer (1999). In order to produce rotational diagrams, we used the total flux over 25 spaxels as well as the decomposed fluxes for the two unresolved gas components (accidentally) located at the positions of MM(A) and MM(B). The two unresolved gas components are designated as (A) and (B) hereafter. (Here, we note again that the two gas components, (A) and (B) are not necessarily associated with the two continuum sources, MM(A) and MM(B).)

The ISO beam and the HIFI beam were too big to resolve (A) and (B) at all. However, due to the much better resolution of PACS, we could decompose the fluxes of (A) and (B) to produce the rotation diagrams for both gas components separately. The errors of the total flux extracted from the whole 25 spaxels are listed in Table 1 while those of the decomposed fluxes are assumed to be 20% of the fluxes. Since we do not know the actual emitting area of each molecule, we use the total number of molecules ($\mathcal {N}$) instead of column density. The results of our rotational diagram analysis are summarized in Table 2.

Table 2. Summary of the Rotation Diagram Analysis

Species	Component	5 × 5		(A)		(B)
		Trot	$\mathcal {N}$(molecule)a	Trot	$\mathcal {N}$(molecule)a	Trot	$\mathcal {N}$(molecule)a
		(K)	(1047)	(K)	(1047)	(K)	(1047)
CO	Hot	758 ± 54	65 ± 19	854 ± 42	55 ± 10	455 ± 37	31 ± 13
	Warm	293 ± 30	275 ± 98	314 ± 24	164 ± 41	250 ± 15	136 ± 34
H2O	Para	168 ± 6	0.06 ± 0.01	150 ± 5	0.06 ± 0.01	74 ± 3	0.06 ± 0.01
	Ortho	144 ± 5	0.24 ± 0.03	154 ± 4	0.15 ± 0.02	88 ± 2	0.13 ± 0.02
OH	2Π3/2-2Π3/2	115 ± 9	0.076 ± 0.020	136 ± 9	0.044 ± 0.008	...b	...b
	2Π1/2-2Π1/2	114 ± 7	0.040 ± 0.013	120 ± 4	0.027 ± 0.005	...b	...b

Notes. ^aThe total number of molecules. ^bThe number of data points for this fitting is not large enough for a meaningful result.

Download table as:  ASCIITypeset image

4.1.1. CO

The rotational CO ladder seems to contain a break at E_u∼ 1500 K (Figure 17). Therefore, we fitted the rotational diagram with two components. High-J CO lines (E_u > 1500 K) are fitted by T_rot of 750–850 K and $\mathcal {N}$(CO) ∼6 × 10⁴⁸ for the total fluxes and the fluxes of (A) while they are fitted by a lower T_rot of ∼450 K with $\mathcal {N}$(CO) ∼3 × 10⁴⁸ for the fluxes of (B). (According to Green et al. (2013), the break position between the two components does not affect the result much.) Low-J CO lines (E_u< 1500 K) are fitted by a T_rot of ∼300 K for the total fluxes measured over all 25 spaxels, as well as for the fluxes of (A). In contrast, the fluxes of (B) are fitted by a lower T_rot of ∼250 K. (The difference in the rotation temperature between (A) and (B) is much greater than the fitting errors.) Therefore, the gas at (A) seems hotter than the gas at (B). The total numbers of CO molecules for this warm component are much greater than those for the hot component; $\mathcal {N}$(CO) ∼ 2.8× 10⁴⁹, 1.6× 10⁴⁹, and 1.4 × 10⁴⁹ for the total fluxes, (A), and (B), respectively (see Table 2).

Zoom In Zoom Out Reset image size

The warm (T ∼ 300 K) and hot (T ∼ 1000 K) CO gas components have been explained by a combination of photodissociation regions (PDRs) and shocks (Visser et al. 2012). According to the scenario, the UV radiation from the central object can heat the outflow cavity walls up to 300–400 K, but shocks are necessary to heat the gas to emit at high-J CO transitions of E_u > 1500 K. However, recent PACS surveys of YSOs (Manoj et al. 2013; Karska et al. 2013) show that the UV heating along the cavity wall is a minor contributor to the excitation of the FIR CO fluxes. In addition, a more self-consistent two-dimensional PDR model developed by S.-H. Lee et al. (in preparation) also shows that the UV heated outflow cavity wall cannot produce the rotational temperature of 300 K, which is universally fitted by the PACS low-J CO lines (E_u< 1500 K), especially in Class 0 sources. Therefore, shocks seem to be the predominant heating source in these embedded YSOs.

4.1.2. H₂O

4.1.3. OH

According to the rotation diagrams, the gas component (A) located at the central spaxel has higher rotational temperatures and total molecular numbers compared to the gas component (B) located at the spaxel, S. However, the rotational diagrams cannot provide detailed information on kinetic temperatures and densities of the two gas components. In addition, we know that those spatially distinct gas components have different kinematical components based on earlier studies (Bachiller et al. 1990; Dutrey et al. 1996; Nisini et al. 2000, 2013; Kristensen et al. 2011) although our PACS spectrum does not resolve the complex kinematics.

Therefore, we used a non-LTE LVG model, RADEX (van der Tak et al. 2007), to connect different physical conditions to the two spatially distinct components. In the model, the level populations are determined by three physical parameters: the gas temperature $T \rm _K$, the H₂ density n(H₂), and the column density of a molecule divided by the line width, Δv. With RADEX, we explore a wide range of physical conditions to interpret observed line ratios. For molecular data, the LAMDA database (Schoier et al. 2005) was used (Offer et al. 1994; Faure et al. 2007; Yang et al. 2010).

First, we upgraded RADEX⁸ with the subroutine, "newt" (Press et al. 1992; Yun et al. 2009), which is a globally used convergent Newton method, because the downloadable RADEX code sometimes does not easily converge to the solution for H₂O and OH lines. Second, for the OH and H₂O molecules, we tested the importance of radiative pumping by replacing a part of the cosmic background radiation with a blackbody radiation field emitted from the inner boundary of the envelope: ${\rm BACK}=W \times {\rm BB}({T})+\it (1-W)\times {\rm BB}\rm {(2.7K)}$, where BB(T) is the Planck function of temperature T, and W is the filling factor of an inner source. Kristensen et al. (2012) adopted the inner boundary temperature of 250 K from Jørgensen et al. (2002) for their best-fit envelope model of L1448-MM with a power-law density structure described with $n=n_{0}\times (\frac{r}{r_0})^{-1.5}$ (n₀ = 1.3 × 10⁹ cm⁻³, r₀ = 20.7 AU). (The temperature at the inner boundary is not well constrained because it has been derived based on the continuum at the wavelengths of 60 μm to a few millimeters, which is dominated by the outer cold envelope.) We assumed that the radiation emitted from the inner boundary of the envelope traveled through the outflow cavity to reach a position in the outflow cavity wall without much attenuation. If we assume a characteristic density at the position where the radiation lands, the radius (r) from the center to that position can be found from the envelope density profile. Therefore, W = (1 − cosθ) × (1/2), where θ = sin⁻¹(r₀/r), assuming a spherical central radiation source. Finally, the observed intensity is $I=[{\rm BB}(T\rm _{ex}) - \it {\rm BB} \rm (2.7 \,K)] \times (1- \it {\rm exp}(-\tau))$, where $T\rm _{ex}$ and τ are the excitation temperature and the optical depth for each transition, respectively; we assumed that the IR source is not located along the line of sight when deriving line fluxes since the contribution of the flux affected by the IR source (through absorption) to the total flux within a spaxel is negligible.

4.2.1. CO

According to the rotation diagram, CO fluxes of both (A) and (B) seem to require multiple gas components, so we tested three combinations of physical components of gas (one gas component, two gas components, and the gas with a power-law temperature distribution) in order to check whether the non-LTE calculation also shows the same conclusion. We fitted the observed PACS fluxes of (A) and (B), separately, assuming that all gas components contribute to the total flux equally. In this test, we adopted the line width (60 km s⁻¹) of the broad component, which was detected in the HIFI observations (Kristensen et al. 2011) and contributed dominantly to the HIFI fluxes. In order to find the best model, we compared flux ratios since we do not know the actual size of the line emitting source. We scaled total model flux to total observed flux and calculated reduced χ². The best-fit models for three different combinations of gas conditions are summarized in Table 3.

Table 3. The Best-fit LVG Model Parameters for Three Different Combinations of Gas Components

Species	Spatial	One Component				Two Components				Power Law
		Tkin	n(H2)	N(mole)	χ2	Tkin	n(H2)	N(mole)	χ2	n(H2)	b	N(mole)	χ2
	Component	(K)	(cm−3)	(cm−2)		(K)	(cm−3)	(cm−2)		(cm−3)		(cm−2)
CO	A	5000	105	1.9 × 1014	2.41	5000	106	1.9 × 1011	0.92	2.51 × 107	2.95		1.26
						5000	104	6.0 × 1013
	B	4000	104	1.9 × 1018	0.67	5000	104	6.0 × 1018	0.46	6.31 × 106	3.30		0.73
						2000	107	1.9 × 1016
H2O	A	2000	106	6.3 × 1017	6.58	100	105	6.3 × 1018	4.71	2.51 × 107	0.0	1013	8.38
						5000	108	6.3 × 1014
	B	700	107	6.3 × 1015	7.95	1000	105	6.3 × 1017	4.18	6.31 × 106	3.2	1013	21.00
						300	104	6.3 × 1016
OH	A	125	2 × 108	5.0 × 1017	4.67

Download table as:  ASCIITypeset image

According to Neufeld (2012), the PACS CO data can be fitted by one component with a high temperature (T_K∼ 3000 K) and low density (n ∼ 10^4–5 cm⁻³). However, the modeled rotation diagram of (A) with one component shows lower curvature compared to the observed one although the CO fluxes of (B) seems fitted well with the gas model with T_K= 4000 K and n = 10⁴ cm⁻³ (see Figure 18).

Zoom In Zoom Out Reset image size

The two gas components can fit better the CO fluxes of both (A) and (B); the two temperatures for (A) are both 5000 K while the two temperatures for (B) are 5000 K and 2000 K. In the model with a power-law temperature distribution, (A) and (B) have a similar power index, b (∼3), but the density for (A) is about four times higher compared to (B). This test shows that multi-components of gas can explain better the fluxes of both (A) and (B), and the high temperatures derived from the LVG models indicates shock origin.

4.2.2. H₂O

We also tested the three different combinations of gas components for the H₂O lines adopting the line width of 50 km s⁻¹, which is the velocity of the broad component detected by Kristensen et al. (2011), for all components. As for CO lines, the two-component model fits better than the single-component model for both (A) and (B) although the single-component model still fits the observed fluxes reasonably (Figure 19). In the single-component model, (A) requires a higher kinetic temperature (2000 K) than (B) (700 K) while the density for (A) is lower than that for (B). The best-fit model with a power-law temperature distribution for (A) has b = 0, indicative of hot gas components are dominant emitter for the water lines. However, the χ² of this model is much worse than the one- or two-component model because this model assumed that all lines are optically thin. The parameters for these best-fit models are summarized in Table 3.

Zoom In Zoom Out Reset image size

4.2.3. OH

For OH, we fitted fluxes only of (A) since the majority of emission is from spaxel C, except for the 119 μm doublet. In addition, the collision rates for OH are available only up to T = 300 K. As a result, we modeled OH fluxes with a single physical component.

We explored models within the following parameter space: 50 <T < 250 K, $10^3 < \it n(\rm H_2) < 1.3\times 10^{9}$ cm⁻³, and 5× 10¹¹ < N(OH) < 5 × 10¹⁷ cm⁻². We assumed a line width of 50 km s⁻¹, which is appropriate for the broad component of H₂O gas (Kristensen et al. 2011). Since it has been suggested that the FIR radiation field can play an important role in the excitation of OH, we included it in our model, as described in the first part of this section. Although we considered only attenuated emission from the central source as the FIR radiation source without considering the FIR radiation from the surrounding material in situ, the model including the effect of the FIR radiation in the OH excitation can fit the observed fluxes better.

Figure 20 (left) presents our best-fit OH model, where the FIR radiation plays a role in the excitation of OH. In the model, the temperature is 125 K, the H₂ density is 2×10⁸ cm⁻³, and the OH column density is 5×10¹⁷ cm⁻². As described in the very first part of this section, this density would be reached at a radius of 72 AU in the adopted one-dimensional density profile. Therefore, the 250 K blackbody radiation field is diluted to the position of the envelope. To examine the effect of FIR radiation on OH fluxes, we compared the same model without the FIR radiation effect in Figure 20 (right). At lower energy levels, the radiation effect is not prominent, but it makes a significant difference at the highest energy level transition; the flux at the highest energy level (E_u = 875 K) in the model with IR-pumping is greater by a factor of five compared to the flux derived from the model without IR-pumping.

Zoom In Zoom Out Reset image size

Although the IR effect is important for the high energy level transition, the overall fluxes are not affected much. The difference between y-intercepts of ²Π_3/2 and ²Π_1/2 ladders, therefore, in the rotational diagram should have other causes. Figure 21 shows the excitation temperature and optical depth of each transition in the best-fit model. The kinetic temperature of the model is marked as a blue dashed line in the left box. All transitions are sub-thermal, and ²Π_3/2 lines are extremely optically thick compared to ²Π_1/2 transitions. Therefore, the higher optical depth of ²Π_3/2 compared to ²Π_1/2 results in the separation (i.e., different y-intercepts) of two ladders in the rotational diagram, where a constant temperature (T_rot) and the optically thin case are assumed. Figure 21(c) shows the actual level populations of the best-fit model, which results in equal populations for the two spin states. Therefore, the separation of two ladders in the rotational diagram is caused both by optical depth effect and IR-pumping.

Zoom In Zoom Out Reset image size

Based on the ISO observations, Nisini et al. (2000) concluded that the CO, H₂O, H₂, and [O i] IR emission in L1448-MM is caused by a non-dissociative shock with a low velocity, and the kinetic temperature of the shocked gas is about 1200 K. The CO and H₂O line profiles are also very broad (FWHM ∼50 km s⁻¹), strongly suggesting that the associated gas is related to shocks (Kristensen et al. 2011; Nisini et al. 2013). In addition, interferometric observations and analysis of the EHV features in L1448-MM show that they are likely jet-shock features (Hirano et al. 2010). Therefore, the derived excitation conditions and resolved kinematical components infer that shocks play an important role in L1448-MM.

The shocks produced by the interaction between the outflow and the envelope heat the gas, resulting in emission. Since the shock could dissociate the gas in the dense envelope, which consists mainly of molecular gas, the shock could change relative abundances among the species H₂, H, O, CO, and H₂O. Therefore, the relative intensities of these emission lines are indicative of the type and speed of the shock waves and of the physical conditions in the gas. We compared our line fluxes to the calculation by Flower & Pineau des Forêts (2010) for both C- and J-shocks. In the comparisons, we consider a single physical component for all the emission. The calculations by Flower & Pineau des Forêts (2010) cover shock velocities from 10 to 40 km s⁻¹ (the shock with 40 km s⁻¹ is only for the C-shock) and hydrogen densities n(H) of 2 × 10⁴ ∼ 2 × 10⁵ cm⁻³. The magnetic field strength in the pre-shock gas is given as b[n(H)(cm⁻³)]^1/2 μG, with b = 1 in the C-shock and b = 0.1 in the J-shock.

The relative CO line fluxes of (A) are well matched by the J-shock model with n(H₂) = 2 × 10⁴ cm⁻³ and v = 20 km s⁻¹. The C-shock model with n(H₂) = 2 × 10⁵ cm⁻³ and v = 40 km s⁻¹ also reproduces the observed line flux ratios reasonably well. In the case of (B), the relative line fluxes are well fitted by the C-shock model with n(H₂) = 2 × 10⁵ and v = 40 km s⁻¹ (Figure 22). The diameter of emitting areas derived from these models are about 1000–2000 AU.

Zoom In Zoom Out Reset image size

The relative H₂O line fluxes of (A) are well matched by the C-shock model with n(H₂) = 2 × 10⁵ cm⁻³ and v = 40 km s⁻¹. For (B), the C-shock model with n(H₂) = 2 × 10⁴ cm⁻³ and v = 20 km s⁻¹ and the J-shock model with n(H₂) = 2 × 10⁵ cm⁻³ in v = 20 km s⁻¹ and 30 km s⁻¹ fit well the observed flux ratios (Figure 23). When we consider that H₂O line fluxes are highly scattered, (B) can be also explained by the shock model for (A). In that case, the relative fluxes of CO and H₂O at both (A) and (B) are reproduced by the C-shock model with n(H₂) = 2 × 10⁵ and 40 km s⁻¹, supporting the idea that H₂O and CO are excited at the same physical conditions as suggested in Karska et al. (2013).

Zoom In Zoom Out Reset image size

Due to its low upper level energy of 230 K, the [O i] 63 μm line is easily excited, and thus, it is an important cooling channel in the post-shock region. As a result, it is one of the best tracers of shocks in dense environments (Giannini et al. 2001). We calculated the [O i] 63/145 μm ratio for (A) and (B), which are ∼22 and 9, respectively. In (A), flux ratio corresponds to low density C-shock model while flux ratio of (B) fits to a high density C-shock model as seen in Figure 24 (left). Therefore, the flux ratios of [O i] lines are consistent with the C-shock model.

Zoom In Zoom Out Reset image size

However, the [O i] flux is severely underestimated by the C-type shock models; an emitting region of >10⁴ × 10⁴ AU², corresponding to >50 spaxels, would be required. Alternatively, more than 50 individual shocks would be required to generate this amount of emission. If a J-shock model is adopted (with v = 30 km s⁻¹ and n(H₂) = 2 × 10⁴ cm⁻³ for (A) and 2 × 10⁵ cm⁻³ for (B)), the emitting area is 300 × 300 AU² (1/40 spaxel) for (A) and 400 × 400 AU² (1/25 spaxel) for (B). Considering the spatial distribution of the [O i] emission covering more than four spaxels (Figure 15), neither a single C-shock nor a single J-shock can reproduce the [O i] absolute fluxes and extent, indicative of multiple shock components.

Flower & Pineau des Forêts (2010) also calculated several H₂ line strengths under the same shock conditions and compared the intensities with those of [O i] emission. In (A), the ratio is fitted to a J-shock model with shock speed of 20–30 km s⁻¹ and a low density C-shock model with $v\rm _{shock} <$ 20 km s⁻¹ simultaneously (Figure 24). In (B), however, the observed fluxes cannot be compared with the model flux ratios since the H₂ fluxes avoids the (B) position. Therefore, the ratio of [O i] to H₂ emission could not constrain the shock characteristics in (B).

These comparisons suggest that a C-shock model can explain most of emission in L1448-MM, but the [O i] absolute fluxes and the [Si ii] emission detected in the IRS spectra indicate that a dissociative J-shock should also exist in this region. According to Neufeld & Dalgarno (1989), [Si ii] emission cannot be produced by a non-dissociative shock, but could be produced by either a dissociative shock or PDR. The dissociative shock tracers such as [Si ii] and [Fe ii] often go close together with non-dissociative shock tracers (e.g., Neufeld et al. 2009), and the dissociative apex of a bow shock that is flanked by non-dissociative shocks is suggested to explain this feature. Therefore, we cannot designate a single type of shock in the simple planar models for L1448-MM; we require more sophisticated multi-dimensional shock models with different initial conditions to explain the relative emission of each species in this complicated region.

Table 4 presents luminosities for the lines detected in L1448-MM as well as continuum. The total line luminosity accounts for only ∼0.7% of the total FIR luminosity in the PACS range. Therefore, the dominant cooling occurs by the continuum radiation. According to Nisini et al. (1999) and Giannini et al. (2001), the FIR line luminosities of L1448-MM calculated from the ISO observations are greater than what we derive from our PACS observations by factors of two to eight depending on species although the relative luminosities among different species are similar. However, ISO and PACS both show that the line cooling mainly occurs through H₂O emission. In contrast, the FIR continuum luminosity obtained by ISO is much smaller than what derived by PACS by a factor of 20, resulting in a higher fraction of line luminosity to the continuum luminosity in FIR. These differences in luminosities between ISO and PACS are probably caused by the low sensitivity and large beam of ISO compared to PACS. In L1448-MM, molecular emission extends beyond the FOV of PACS. Therefore, the ISO beam, larger than the PACS FOV, possibly picked up a significant amount of the extended molecular emission.

Table 4. Luminosities in FIR^a for the Full Map and Each Position

Species	5 × 5	(A)	(B)
LCO	0.96	0.70	0.27
$L_{\rm H_2O}$	1.73	1.21	0.39
LOH	0.34	0.30	0.05
LOI	0.17	0.07	0.04
Lcontinuum	430	349b	90b

Notes. ^aIn units of 10⁻² L_☉. ^bThe FIR luminosities of MM(A) and MM(B).

Download table as:  ASCIITypeset image

In the PACS line luminosity, CO, H₂O, and OH emission arises mostly from (A); ∼70% of the total CO and H₂O fluxes are emitted from (A), while (B) supplies ∼30% of the CO and H₂O line cooling, and most of OH line emission (∼90%) is concentrated on (A). However, the sum of [O i] line luminosity of (A) and (B) has just ∼65% of the total luminosity calculated over the whole 5 × 5 spaxels, indicative of broadly extended emission in [O i]. If we assume that the gas components of (A) and (B) are associated with MM(A) and MM(B), respectively, the ratio of the FIR line luminosity in the PACS range to the bolometric luminosity (L_mol/L_bol) both for MM(A) and MM(B) is ∼4 × 10⁻³, indicating that both sources are in the Class 0 stage. According to Giannini et al. (2001), L_mol/L_bol > 1 × 10⁻³ for Class 0 objects while the ratio is smaller than 5 × 10⁻⁴ for Class I and II sources. According to our best-fit LVG models, H₂O and CO emit the majority (>50%–80%) of their luminosity in the PACS wavelength range.

Table 5 shows the fractional contribution of each species to the FIR line luminosity as a total and in (A) and (B). van Dishoeck et al. (2011) suggested that H₂O might not be the dominant coolant in YSOs. For L1448-MM, however, water is the primary outflow coolant and this is consistent with the results for the Class 0 protostar NGC 1333 IRAS 4B (Herczeg et al. 2012), where H₂O is responsible for 72% of line luminosity. H₂O occupies ∼50% and CO fills ∼30% of the line luminosity in the PACS wavelength range in L1448-MM. For CO, the warm ($E\rm _{u}<$ 1500 K) and hot ($E\rm _{u} >$ 1500 K) components are responsible for ∼65% and ∼35% of the CO line luminosity in the PACS range, respectively, based on the results of rotation diagrams. OH and [O i] contribute ∼10% and ∼5% of the line luminosity, respectively. In L1448-MM, the cooling through molecular emission is significantly larger than the cooling via atomic emission, which is consistent with the characteristic of the Class 0 YSOs (Nisini et al. 2002; Herczeg et al. 2012). Bright H₂O emission and dim [O i] emission in L1448-MM is indicative of low dissociation rate of H₂O, or the fast formation process of H₂O in the post-shock gas. Alternatively, [O i] and H₂O emission possibly arises from unassociated gas components, i.e., the [O i] emission is attributed to a more extended gas while the H₂O emission is localized around the YSOs.

Table 5. Fractional Contribution to Total Line Cooling^a for Full Map and Each Position

Species	5 × 5	(A)	(B)
LCO	30%	31%	36%
$L_{\rm H_2O}$	54%	53%	52%
LOH	11%	13%	7%
LOI	5%	3%	5%

Note. ^a$L_{\rm total, line}=L_{\rm H_2O}+L_{\rm CO}+L_{\rm OH}+L_{\rm OI}$.

Download table as:  ASCIITypeset image

Previously, L1448-MM was known as a single YSO with a prominent outflow. However, Jørgensen et al. (2007), Tobin et al. (2007), and Hirota et al. (2011) suggested the existence of a secondary YSO, named L1448-MM(B) while Maury et al. (2010) suggested that the secondary point-like source at Spitzer bands and 3 mm might be a result of a shock on the outflow cavity wall. However, we conclude that L1448-MM(B) is a YSO based on its millimeter SED (Figure 2) and the detection of the double peak structure of CO₂ ice absorption feature at 15.2 μm (Figure 4) (Pontoppidan et al. 2008).

The SEDs (Figure 2) of MM(A) and MM(B) rising into the FIR indicate that both sources are very embedded. Hirano et al. (2010) suggested that MM(B) was likely less obscured in the MIR compared to MM(A). This is consistent with a much higher FIR flux level in MM(A) than in MM(B), indicating less material toward MM(B). From the separated SEDs and the photometric data points in Green et al. (2013), we calculated the bolometric luminosities (L_bol) of MM(A) and MM(B) as 5.5 and 1.7 L_☉, respectively, which do not add up to L_bol of 8.4 L_☉ calculated over the whole region (Green et al. 2013). For the calculation, single-dish (sub)millimeter fluxes for two sources were separated based on the flux ratios derived from the interferometric observations. (The interferometric continuum fluxes were not included in the calculation of L_bol and T_bol although they do not affect the results at all.) The derived L_bol for each source is rather sensitive to this flux separation at (sub)millimeter. Therefore, it should not be considered very accurate. The calculated bolometric temperatures (T_bol) of MM(A) and MM(B) are 49 and 80 K, respectively, indicating that MM(A) is more embedded and in the earlier evolutionary stage. L_bol/L_smm are 18.7 and 48.7 for MM(A) and MM(B), respectively, which also suggests that MM(A) is more embedded. Note that MM(B) is classified as Class I by T_bol but Class 0 by L_bol/L_smm while MM(A) is classified as Class 0 by both criteria.

MM(A) is separated from MM(B) by 817 that is equivalent to ∼2000 AU. Mundy et al. (2001) divided embedded multiple systems into three groups: independent envelope, common envelope, and common disk systems. As seen in the submm continuum maps (Shirley et al. 2000), two sources seem associated with only one dense core. Therefore, L1448-MM may be a common envelope system which has one primary core in gravitational contraction, and where objects are separated by 250–3000 AU. Although the possible detection of the CO₂ gas line at 14.98 μm (Dartois et al. 1999) toward MM(A) is indicative of a hotter region, the column density of CO₂ ice, calculated from the IRS 15.2 μm CO₂ ice feature, is ∼2 × 10¹⁸ cm⁻² in both YSOs. This might also support the idea that they are in a common envelope.

The outflow activity in the two sources appears significantly different. Hirano et al. (2010) has reported that the weak CO outflow possibly associated with MM(B) is nearly perpendicular to that of MM(A), and its small momentum flux is comparable to those of outflows by Class I objects. However, the MM(A) outflow elongates in the SE–NW direction. Since MM(B) is about 8'' south of MM(A), fluxes in spaxel S are likely contaminated by the outflow emission from the MM(A). The continuum emission of MM(A) is greater than that of MM(B) at λ ⩾ 20 μm. Lines of high energy levels also peak in the position of MM(A) while the emission peaks of lines of lower energy levels shift to the south. These features suggest that MM(A) may be the primary outflow source in the direction of SE–NW. Then the gas traced in our PACS observations may be heated predominantly by the jet/outflow driven by MM(A). If this is true, the molecular gas in (A) and (B) rather correspond to the blue and red wings of the L1448-MM(A) jet, which were detected by the CO and SiO transitions (Nisini et al. 2000, 2007; Hirano et al. 2010).

According to our excitation analyses of (A) and (B), the PACS FIR line fluxes are possibly produced mainly by shocked hot gas components, rather than by the UV-heated gas along the outflow cavity. According to Yildiz et al. (2010), up to J = 10–9, the contribution of the broad wing component increases with J, hinting that shocks may play a more important role in higher J transitions traced by the PACS. However, if the UV photons play an important role in this region, we should expect an enhancement of the [O i] and OH line fluxes compared to H₂O. In order to check the relative emission among [O i], OH, and H₂O, as done in Lindberg et al. (2013), we calculated the flux ratios between the OH (84 μm, E_u = 291 K) and o-H₂O (75 μm, E_u = 305 K) lines, which have small PSFs and similar upper-level energies, as well as the flux ratios between [O i] (63 μm, E_u = 227 K) and o-H₂O (66 μm, E_u = 410 K) lines, toward the DIGIT embedded sources (Figure 25). According to the comparisons, L1448-MM has the minimum flux ratios among the DIGIT embedded sources, supporting that the observed line emission in L1448-MM is mainly from shocks.

Zoom In Zoom Out Reset image size

Comparing to the recently observed Class 0 objects, NGC 1333 IRAS4B (hereafter IRAS4B) and Serpens SMM1 (hereafter SMM1), L1448-MM is similar to IRAS4B in molecular emission; water dominates in the line cooling and [O i] is dim. In addition, the best model for IRAS4B with the non-LTE analysis suggests the physical conditions of T_K ∼ 1500 K and n(H₂) ∼3 × 10⁶ cm⁻³. In SMM1, however, CO is the main coolant and [O i] is relatively brighter than the other two sources, and the derived physical conditions are T_K ∼ 800 K and n(H₂) ⩾ 5 × 10⁶ cm⁻³. Therefore, the derived physical conditions for L1448-MM are more similar to those of IRAS4B. In addition, [C ii] (158 μm) is detected in SMM1 but not detected in IRAS4B and L1448-MM. L_OH/$L_{\rm H_2O}$ of L1448-MM and IRS4B are ∼0.2 while that of SMM1 is 0.4. L_OI/$L_{\rm H_2O}$ are 0.06∼0.1, 0.01, and 0.6 for L1448-MM, IRS4B, and SMM1, respectively. Goicoechea et al. (2012) mentioned that the strong [O i] and OH emission toward SMM1 is similar to that of HH46, a Class I source (van Kempen et al. 2010a). Therefore, the FIR line emission in L1448-MM and NGC 1333 IRAS4B might have a similar origin, i.e., non-dissociative shocks shielded from UV radiation play a larger role in the excitation than dissociative shocks (Herczeg et al. 2012).