Open Access
Issue
A&A
Volume 630, October 2019
Article Number A136
Number of page(s) 13
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201935568
Published online 07 October 2019

© Z. Nagy et al. 2019

Licence Creative Commons
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Open Access funding provided by Max Planck Society.

1 Introduction

Starless cores with temperatures of about 10 K and densities above 104 cm−3 define the stage just before the onset of low-mass star formation in molecular clouds. When the central H2 density of the cores is above 105 cm−3, starless cores become thermally supercritical and start to collapse. These are the so-called pre-stellar cores. Probing the physics and chemistry of such regions is essential for understanding the process of star formation. The chemistry of starless and pre-stellar cores can be traced by different molecular lines and the dust continuum at millimetre and sub-millimetre wavelengths. In this paper we aim to characterize the chemical structure of a starless core in an early evolutionary phase.

The L1521E starless core is located in the Taurus region, at a distance of 145 pc (Yan et al. 2019). It was classified as a very young core by Hirota et al. (2002) because of the high abundances of carbon-chain molecules, comparable to those in TMC-1 (Aikawa et al. 2003). Tafalla & Santiago (2004) further confirmed that L1521E was a very young core as they found no evidence of C18O depletion. They estimate its age to be ≤1.5 × 105 yr. Ford & Shirley (2011) confirmed the low level of CO depletion by fitting C18O line profiles using a non-LTE radiative transfer code. Hirota et al. (2002) derived the peak H2 volume density to be (1.3−5.6) × 105 cm−3, which is consistent with the value of 2.7 × 105 cm−3 found by Tafalla & Santiago (2004). Based on Herschel/SPIRE observations, Makiwa et al. (2016) measured the dust temperature to be 9.8 ± 0.2 K and the core mass to be 1.0 ± 0.1 M.

In this paper we aim to study the chemical structure and CO depletion of L1521E and compare it to the more evolved and more massive pre-stellar core L1544. L1544 is a well-studied pre-stellar core also located in Taurus (Spezzano et al. 2017 and references therein). Based on a comparison of observations and chemical models of simple deuterated species toward L1544, Kong et al. (2015) derived an age of a few 105 yr. Based on 1.2 mm contimuum data, the N(H2) toward the dust peak is (9.4 ± 1.6) × 1022 cm−2 and the H2 volume density within a radius of (3.2 ± 0.4) × 103AU is (1.4 ± 0.2) × 106 cm−3 (Crapsi et al. 2005; Chacon-Tanarro et al. 2019). Comparing the less evolved L1521E to the more evolved and better characterized L1544 will help us to gain understanding of the evolution of dense cores, and hence the process of star formation. The paper is organized as follows: we describe the observations in Sect. 2 and in Sect. 3. Results on the spatial distribution and column densities of the molecules, and on CO depletion are presented in Sect. 4. We discuss the results and compare them to those toward L1544 in Sect. 5 and summarize them in Sect. 6.

thumbnail Fig. 1

Spatial distributions of molecules detected toward L1521E (black contours) overplotted on N(H2) map (colors) derived from Herschel/SPIRE. The black dot, triangle, and asterisk show the c-C3H2, CH3OH, and HNCO peaks, respectively. The black square shows the Herschel dust peak. The beam size overplotted on the SO map shows the beam size of the SPIRE data, for the other maps it is the beam size of the IRAM-30 m data. For SO, CS, 13CS, C2S, and C34S the contour levels start from 3σ rms in steps of 3σ rms with 3σ rms values of 0.13, 0.38, 0.06, 0.12, and 0.11 K km s−1, respectively. For OCS and C33S the contour levels start from 6σ rms in steps of 4σ rms with 3σ rms levels of 0.02 K km s−1 for both species. For 34SO, C3S, CC34S, H2CS, and HCS+ the contour levels start from 9σ rms in steps of 6σ rms with 3σ rms values of 0.01, 0.02, 0.01, 0.07, and 0.03 K km s−1, respectively.

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thumbnail Fig. 1

continued. For C17O, CN, H13CN, HCN, and HCO+ the contour levels start from 3-σ rms in steps of 3-σ rms with 3-σ rms values of 0.05, 0.05, 0.04, 0.17, and 0.34 K km s−1, respectively. For N2H+, HN13C, HNCO, HC3N, and HC18O+ the contour levels start from 6-σ rms in steps of 4-σ rms with 3-σ rms levels of 0.08, 0.08, 0.05, 0.12, and 0.02 K km s−1, respectively. For CH3CN and CH3CCH the contour levels start from 9-σ rms in steps of 6-σ rms with 3-σ rms values of 0.01 and 0.03 K km s−1, respectively.

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thumbnail Fig. 1

continued. For H2CCO and HCO the contour levels start from 6-σ rms in steps of 4-σ rms with 3-σ rms levels of 0.02 K km s−1 for both species. For CH3CHO, C2H, C4H, and C4H2 the contour levels start from 9-σ rms in steps of 6-σ rms with 3-σ rms values of 0.01, 0.07, 0.03, and 0.004 K km s−1, respectively.

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2 Observations and data reduction

Maps of various molecules were observed using the Eight MIxer Receiver (EMIR) instrument of the IRAM-30 m telescope in on-the-fly mode with position switching. The Fourier transform spectrometer (FTS) was used as the backend with a spectral resolution of 50 kHz. The 4–6.25 square arcminute maps were centered on the RA(J2000) = 04h29m15.7s Dec(J2000) = + 26°14′5″ position, based on Tafalla & Santiago (2004). Part of the observations were carried out in June and August 2016 (PI: S. Spezzano), and additional observations were done in 2018 January and April (PI: Z. Nagy). The data were reduced using the GILDAS software (Pety 2005). The measured intensities were converted to main beam temperature units using a forward efficiency of 0.94 and a main beam efficiency of 0.78 around the frequency of 115 GHz, and a forward efficiency of 0.95 and a beam efficiency of 0.81 around the frequency of 86 GHz. At these frequencies, the angular resolution of the IRAM-30 m observations is around 30″. When creating the data cubes analyzed in this paper we re-gridded the maps to a pixel size of 4″, as done by Spezzano et al. (2017).

We use Herschel/SPIRE (Griffin et al. 2010) data from bands at 250, 350, and 500 μm to derive the H2 column densities. These data were downloaded from the Herschel Science Archive and are part of the Herschel Gould Belt Survey (André et al. 2010). We fit a modified blackbody function with a dust emissivity index of β = 1.5 to each pixel. The 250 and 350 μm images were smoothed to the resolution of the 500 μm image (~40″). The images were resampled to the same grid.

In addition to the IRAM-30 m and Herschel/SPIRE data we also used the 1.2 mm continuum data from the work of Tafalla &Santiago (2004). These data were measured with the MAMBO-2 instrument of the IRAM-30 m telescope in on-the-fly mode. The spatial resolution of the 1.2 mm continuum data is 11″.

3 Dataset

The datasetanalyzed in this paper includes maps of molecular emission from 30 species (including isotopologs) detected toward L1521E as shown in Table A.1. Some species, such as isotopologs containing 15N and deuterated species will be analyzed in a forthcoming paper. The quantum numbers, frequencies, Einstein A coefficients, and upper level energies are from the Cologne Database for Molecular Spectroscopy (CDMS, Müller et al. 2005)1 and the Jet Propulsion Laboratory (JPL, Pickett et al. 1998)2 molecular databases. We included the critical densities for the transitions whenever collisional rates are available for the molecules. Table A.1 also includes information on whether the transitions detected toward L1521E are detected toward L1544. This information is based on the public data release of the Astrochemical Surveys At IRAM (ASAI, Lefloch et al. 2018; Vastel et al. 2018). A spectrum in the frequency range between 80 and 106 GHz was observed toward the dust peak of L1544, which allows us to directly compare the two sources in Sect. 5. Figure 1 shows integrated intensity maps of the molecules detected toward L1521E, which will be further discussed in the next sections.

4 Results

4.1 Spatial distribution of the molecules

Integrated intensity maps of the detected species are shown in Figs. 1 and 2. The spatial distribution of the different species is further shown as line profiles extracted toward the c-C3H2 and methanol peaks in Fig. 3. Furthermore, shown in Fig. 4 are intensity cuts extracted toward a line that crosses the two molecular peaks, and toward a line that is perpendicular to the former and then crosses the Herschel dust peak. The position of the two cuts is shown in Fig. 2. Similar to what was observed in the more evolved L1544 core (Spezzano et al. 2016), methanol and c-C3H2 peak at different positions around the center of L1521E (Fig. 2). Most species peak where c-C3H2 peaks (RA(J2000) = 04h29m16.0s Dec(J2000) = + 26°14′0.9″), such as C2S, C3S, HCS+, HC3N, H2CS, CH3CCH, and C34S. The CN emission peak is between the Herschel dust peak and the methanol peak (RA(J2000) = 04h29m14.4s Dec(J2000) = + 26°14′17.7″), and considering the Herschel and IRAM-30 m beam sizes, it is consistent with both peaks. N2H+ also peaks near the Herschel dust peak.

Figure 5 shows the spatial distribution of gas traced by the detected species compared to the location of the dust peak traced by the 1.2 mm and the Herschel continuum data. The dust peak based on the Herschel/SPIRE data (RA(J2000) = 04h29m14.0s Dec(J2000) = + 26°13′56″) is slightly offset from the 1.2 mm peak (RA(J2000) = 04h29m15.15s Dec(J2000) = + 26°13′50.8″), but considering the large SPIRE beam, it is consistent with both the 1.2 mm peak and the CH3OH peak. Also, SPIRE is probably probing a different dust population compared to the 1.2 mm data. The distribution of N2H+ is similar tothe distribution of dust, as it is also the case for L1544 (Caselli et al. 1999). C17O shows signs of depletion toward the dust peak, which was also observed for L1544 (Caselli et al. 1999). CH3OH peaks in a region where CO is depleted, but not exactly towards the dust peak or towards the region where the CO freeze-out is the most prominent. The spatial distribution of species shown in Fig. 5 and the depletion of CO is further discussed in Sect. 4.4.

thumbnail Fig. 2

Spatial distribution of c-C3H2 20,2 − 11,1 (white contours) and CH3OH (E2) (black contours). These are shown as detected toward L1521E overplotted on the N(H2) map which was derived from Herschel/SPIRE (color map), showing the large-scale environment of L1521E (top) and the spatial structure of molecular line emission close to the center of the core (bottom). The two overplotted lines show the location of the cuts along which the line intensities shown in Fig. 4 were extracted. The contour levels start from 6σ rms in steps of 6σ rms, with 3σ rms levels of 0.07 and 0.06 K km s−1 for CH3OH and c-C3H2, respectively.

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4.2 Principal component analysis

Another method to study the structure of L1521E and probe correlations between the observed molecular species is the principal component analysis (PCA), which Spezzano et al. (2017) previously performed for L1544. A detailed description of the method is shown in Ungerechts et al. (1997). The PCA explains the total variability of correlated variables through the use of the same number of orthogonal principal components. Here the number of correlated variables is the number of maps used for the analysis: 24. Each map consists of 27 × 27 pixels in a space with 24 dimensions (the number of molecular transitions used). We used a transition for most molecular species included in this analysis, except for the species with the lowest signal-to-noise ratio (S/N): C3S, CC34S, CH3CHO, HC18O+, HCO, OCS, and C4H2. Similar to Spezzano et al. (2017), we performed the PCA on a standardized (mean-centered and normalized) dataset, with the standardized value defined as xstd = (xμ)∕σ, where μ is the mean value and σ is the standard deviation for each map. While it is possible to compute 24 principal components (PCs) from our data, we analyze the first four PCs in this paper, as those account for most of the correlation (91.3%) in the data. Figure 6 shows maps of the first four PCs. The first PC correlates with the continuum emission, as shown in Fig. 1. Similar to the case of L1544 (Spezzano et al. 2017), the second PC shows the two most prominent molecular peaks, the c-C3H2 and CH3OH peaks. The third and fourth PCs account for only 2.9 and 2.4% of the correlation and are likely to be dominated by noise. Figure 7 shows the contribution of each molecule to the four PCs, and Fig. 8 shows correlation wheels where the coordinates for each molecule are given by their correlation coefficients to each PC, obtained by performing the PCA on the standardized data.

The PCA results for L1521E and those presented by Spezzano et al. (2017) for L1544 are similar, as seen in Fig. 6 from this paper and in Fig. 2 from Spezzano et al. (2017). The first PC follows the distribution of the continuum in both cases, however, it represents 78.0% of the correlation in the dataset for L1521E, and only 67% for L1544. The second PC shows the methanol and c-C3H2 peaks in case of both sources, but it represents a higher correlation (12.2%) for L1544 than for L1521E (8.0%). The differences between the results for the two cores may be related to the fact that while 28 different molecular line maps were included in the PCA for L1544, only 24 species were available with a sufficient S/N level toward L1521E.

thumbnail Fig. 3

Line profiles of selected molecules observed toward CH3OH peak, c-C3H2 peak, and Herschel dust peak. The transitions used for each molecule are marked in Table A.1.

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4.3 Molecular column densities

We derived column densities for the observed species toward the Herschel dust peak in an optically thin approximation, assuming a single excitation temperature and negligible background radiation. We measured the excitation temperature by fitting the hyperfine structure of the N2H+ (1–0) line toward the Herschel dust peak, using Gildas/CLASS. The excitation temperature given by the fit is ~4.5 K. We used this excitation temperature to derive the column density of all the species, except for C17O. Due to its low critical density (~2 × 103 cm−3), C17O is supposed to be thermalized, therefore we assumed the excitation temperature for C17O to be 10 K, which is close to the dust temperature of 9.8 K derived by Makiwa et al. (2016). The measured integrated intensities of the lines (∫ TMBdV) can be converted to the column densities of the molecules (Ntot) in optically thin approximation using: (1)

where gu is the statistical weight of the upper level, Q(Tex) is the partition function for Tex, Eu is the upper level energy, ν is the frequency of the used transition, and Aul is the spontaneous decay rate. Eq. (1) also assumes a beam filling factor of 1, which means that the source emission is assumed to be more extended than the beam size. The column densities of the species are summarized in Table 1. The transitions used to calculate column densities are marked with an asterisk in Table A.1. The error of the column densities is dominated by calibration error and error given by the uncertainty of the assumed excitation temperature.

thumbnail Fig. 4

Left: intensity cuts of selected molecules extracted along line crossing the two C17O peaks (close to c-C3H2 and CH3OH peaks) for various species (cut 1 in Fig. 1). Right: intensity cuts extracted along a line perpendicular to other cut and crosses Herschel dust peak (cut 2 in Fig. 1). The intensity values (y-axis) are normalized to the maximum value for each species, and a constant has been added to compare the normalized intensity distributions of the different species. The vertical dotted lines show the locations of the C17O and N(H2) peaks.

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Table 1

Molecular column densities derived toward the dust peaks of L1521E and L1544.

4.4 CO depletion

As already seen in Fig. 5, the C17O line intensities decrease around the Herschel dust peak, which suggests that there is some CO depletion. To calculate the CO depletion factors we used the observed C17O map and the N(H2) map derived from the Herschel/SPIRE data. We use the J = 1−0, F = 5/2−5/2 transition of C17O at 112.36 GHz to derive the C17O column density in optically thin LTE approximation assuming an excitation temperature of 10 K (Tafalla & Santiago 2004). For this analysis we smoothed the C17O map from its original spatial resolution (~30″) to the resolution of the N(H2) map from the Herschel/SPIRE data (~40″). We used a 16O/17O abundance ratio of 2044 (Penzias 1981; Wilson & Rood 1994) and a canonical CO abundance of 8.5 × 10−5 (Frerking et al. 1982).

To derive N(H2) from the Herschel/SPIRE fluxes we used a dust emissivity index of β = 1.5. The value of β = 1.5 gives a good comparison with L1544 as it was used to derive N(H2) for that source (Spezzano et al. 2016). Other studies, such as Kirk et al. (2007) assumed a value of β = 2.0 for L1521E. A β = 2.6 ± 0.9, which was derived from Herschel/SPIRE data by Makiwa et al. (2016) is also consistent with this value.

The calculation for β = 1.5 results in a CO depletion factor of 4.3 ± 1.6 toward the Herschel dust peak. The derived depletion factors are shown in Fig. 9, with the methanol integrated intensity contours overplotted. Even though the depletion factors do not peak at the same position as the methanol intensities, the depletion factors increase with increasing methanol abundance, which was expected as CH3OH is produced onthe surface of dust grains by CO hydrogenation (Watanabe & Kouchi 2002) and then released to the gas phase via reactive desorption (Vasyunin et al. 2017).

We also used the 1.2 mm data to derive N(H2) from a different method described in Crapsi et al. (2005). We smoothed the 1.2 mm map to the resolution of the IRAM-30 m data (~ 30″). For the dust opacity per unit mass we used a value of κ1.2mm = 0.005 cm2 g−1. This value was chosen following Crapsi et al. (2005), who derived depletion factors for a number of starless cores. Originally, André et al. (1996) chose this value based on the recommendation of Henning et al. (1995) and Preibisch et al. (1993) for clouds at intermediate densities (~105 cm−3) and gas-to-dust ratios of 100. The CO depletion factors resulting from this method are similar to the values derived using N(H2) values from the Herschel data, with a value of 3.9 ± 1.4 toward the Herschel dust peak. The large depletion factors based on the 1.2 mm data in the southern part of the map in Fig. 9 are probably an artifact due to the filtering of the extended cloud structure by MAMBO-2. The S/N in these parts of the map is below three.

Crapsi et al. (2005) calculated depletion factors for several starless cores that are in the range between 3.4 ± 2.1 and 15.5 ± 3.7, including the value 14 ± 2.2 for L1544, which is about three times the value derived for L1521E (4.3 ± 1.6) when assuming β = 1.5 for the H2 column density. Based on the comparison of C18O observations of L1521E to chemical models, Tafalla & Santiago (2004) concluded that CO depletion is not present in L1521E, given that the data are consistent with a constant abundance profile. Similarly, Ford & Shirley (2011) found little or no C18O depletion toward L1521E by the comparison of observations and chemical models. Tafalla & Santiago (2004) found some differences between the distribution of the C18O and continuum emission outside the core central region (which is also seen in Fig. 1 from Ford & Shirley 2011), but attributed them to the low sensitivity of the data at those positions, and not to CO depletion. The depletion factor of 4.3 ± 1.6 found using the data presented here and the spatial distribution of C17O (1–0) suggest that some CO depletion is present in L1521E near the dust peak.

thumbnail Fig. 5

Structure of L1521E. Left: integrated intensity of CH3OH (black contours) overplotted on integrated intensity of C17O. The black dot shows the peak of the 1.2 mm emission. The black triangle shows the peak of the H2 column density derived from the Herschel/SPIRE data. The CH3OH contour levels start from 6σ rms in steps of 6σ rms, with a 3σ rms level of 0.07 K km s−1. Right: integrated intensity of N2H+ (black contours) and distribution of 1.2 mm emission (white contours) overplotted on integrated intensity of C17O. The black dot shows the peak of the 1.2 mm emission. The black triangle shows the peak of the H2 column density derived from the Herschel/SPIRE data. The N2H+ contour levels start from 6σ rms in steps of 4σ rms, with 3σ rms level of 0.08 K km s−1. For the 1.2 mm data the contours start from 6σ rms in steps of 3σ rms with a 3σ rms level of 5.38 mJy.

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thumbnail Fig. 6

Maps of first four principal components obtained by performing PCA. The percentages represent the amount of correlation that can be reproduced by the single principal component. The red triangle represents the CH3OH peak, the asterisk is the c-C3H2 peak, and the square the is HNCO peak.

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thumbnail Fig. 7

Contribution of each transition to first two PCs, obtained by performing PCA on the standardized data.

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thumbnail Fig. 8

Correlation wheels where the coordinates for each molecule are their contribution to each PC, obtained by performing the PCA on the standardised data.

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4.5 Comparison to models

To understand the measured column densities, we preformed chemical modeling using the 1D code described in Vasyunin et al. (2017). The model represents the core with a spherically symmetrical structure. We used the density structure reported by Tafalla & Santiago (2004) and a constant temperature profile at T = 10 K, also used by Tafalla & Santiago (2004). In the model we calculated time-dependent fractional abundances and column densities of species. Then the calculated column densities are convolved with a 26″ Gaussian beam, to be comparable to those measured with the IRAM-30 m. In the model, only the main isotopologs of the species are available, therefore, we used isotopic ratios based on Wilson & Rood (1994) to compare the modeled and observed column densities. We ran several models with different initial abundances and compared the resulting column densities to those obtained from the observations, to find a model that fits the most observed column densities within an order of magnitude. The best fit model uses “low metals” (EA1) initial abundances from Wakelam & Herbst (2008), except for the fractional abundance of S+, which needs to be increased to 10−6 instead of 8 × 10−8 to fit the observations. The model was run in two stages. First, we ran the evolution of a diffuse cloud for 106 yr. Then, we took the final abundances of species from the first stage as the initial chemical composition for the next stage, and ran the evolution of L1521E itself. The best fit column densities from the models are shown in Table 2 and in Fig. 10 as a function of radius. This model is able to reproduce most measured column densities, especially those of the sulfur-bearing molecules except for SO. This supports the result that significant sulfur depletion takes place during the evolution of dense cores. The best fit model suggests a chemical age of ~1.7 × 105 years, which is close to what was found by Tafalla & Santiago (2004). For this age estimate t = 0 is the start of the second stage of the model run mentioned above, when the adopted 1D physical structure of the core has been formed. Since our physical model does not evolve with time, the obtained age is rather a timescale of the relaxation for chemical evolution of the model core up to the observed chemical composition rather than an estimate of the physical age of the real core. The CO depletion factor at the moment of best agreement towards the center of L1521E in the model is ~3, which is close to the value of 4.3 ± 1.6 derived from the observations.

thumbnail Fig. 9

Map of CO depletion factors toward L1521E. These were calculated using H2 column densities based on the Herschel/SPIRE data with β = 1.5 (left panel) and based on the 1.2 mm map (right panel), the derived C17O abundances, and a reference value of X(C17O) = 5.4 × 10−8. The contours show the spatial distribution of methanol and start from 6σ rms noise level in steps of 6σ rms. The triangle shows the peak of the 1.2 mm emission and the asterisk symbol shows the Herschel dust peak. The depletion factor toward the Herschel dust peak is about 4.3 based on the Herschel N(H2) values and about 3.9 based on the N(H2) values calculated from the 1.2 mm data. The black circles show the beam sizes that were used to calculate the depletion factors.

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Table 2

Best fit column densities derived for L1521E with chemical code described in Vasyunin et al. (2017) and agreement with measured values given in Fig. 11.

5 Discussion

We analyzed IRAM-30 m observations of various molecules toward the starless core L1521E. We found that, similarly to L1544, CH3OH and c-C3H2 peak at twodifferent positions, but the distance between the two peaks is smaller and similar to the size of the CO depletion zone (in both cases). Most observed species peak at the c-C3H2 peak. The C17O column densities calculated toward L1521E suggest that CO is depleted toward the Herschel dust peak. The depletion factor based on the C17O data and N(H2) calculated from Herschel/SPIRE data was found to be 4.3 ± 1.6 and the depletion factor based on the 1.2 mm emission was found to be 3.9 ± 1.4.

We compared the column density ratios of the species measured toward the dust peak of L1521E with those measured toward the dust peak of L1544. To estimate column densities toward L1544 we used the spectrum measured toward L1544 in the 81–106 GHz range as part of the ASAI (Lefloch et al. 2018). The column densities of the species toward L1544 were derived with the same method as for L1521E described in Sect. 4.3, using the same transitions of the species toward both cores. We assumed an excitation temperature of 5 K which was derived by Crapsi et al. (2005) by fitting the N2H+ (1–0) hyperfine structure. Since the H2 column densities reported toward L1544 define a significant range, from 2.8 × 1022 cm−2 (Spezzano et al. 2016) derived from Herschel/SPIRE data to (9.4 ± 1.6) × 1022 cm−2 derived from the 1.2 mm continuum (Crapsi et al. 2005), we compared column density ratios toward the two cores. It is important to note that the column densities were derived from data measured with similar beam sizes. The column density ratios with the reference column density of CH3OH (A+) toward the two cores are shown in Fig. 11. One of the main differences between the column density ratios calculated toward the two sources is seen for the sulfur-bearing species with carbon, such as C2S, HCS+, C34S, C33S, and HCS+, which have higher abundances toward L1521E than toward L1544. These C- and S-bearing molecules were found to follow the c-C3H2 peak in L1544 (Spezzano et al. 2017), meaning toward L1544 they appear to trace chemically young gas, probably affected by the interstellar radiation field. The relatively higher abundance of these molecules toward L1521E may indicate that sulfur depletion continues to take place during the dynamical evolution of dense cores, as also recently found by Laas & Caselli (2019), therefore, organic S-bearing molecules can be considered good tracers of chemically young dense cores. Among the S-bearing species studied here, SO was also found to trace cores in an early evolutionary state by other studies, such as Hacar et al. (2013), who found extended SO emission toward the L1495/B213 complex in Taurus. They interpreted SO to trace gas in regions of early evolutionary state as it was found to be bright in regions with bright C18O emission and intense dust millimeter continuum together with little N2H+ emission. Tafalla et al. (2006) studied the abundances of molecules toward the L1498 and L1517B starless cores in Taurus. Based on the comparison of observations to radiative transfer models they found SO and C2S to be good tracers of molecular depletion.

The CH3OH (E2) relative abundances are similar in the two cores. The fact that the N2H+ abundance toward L1521E is higher than the abundance toward L1544 confirms that L1521E is indeed in an earlier stage of core evolution, due to the relatively long formation route of N2H+ starting from atomic nitrogen (e.g. Hily-Blant et al. 2010).

In addition to the abundances, the spatial distribution of species helps us to better understand the difference between thetwo cores. Spezzano et al. (2017) found four different molecular peaks toward L1544: dust peak, c-C3H2 peak, methanol peak, and HNCO peak. These peaks are also present toward L1521E, but they are closer together, compared to the more dynamically evolved L1544 pre-stellar core. N2H+ and CN peak close to the dust peak toward L1521E, and similarly N2H+ and 13CN also peak close to the dust peak toward L1544 (Spezzano et al. 2017). Not many species were found to peak toward the methanol peak in L1521E. SO, which peaks toward the methanol peak in L1544 shows two peaks in L1521E, one toward the c-C3H2, and one close to the CH3OH peak. Spezzano et al. (2017) found that O- and S-bearing molecules follow the methanol distribution toward L1544, and therefore trace more chemically evolved gas or gas that is more protected from the interstellar radiation field (see also Swade 1989).

Most species (mostly carbon chain molecules) peak toward the c-C3H2 peak in both cores. One difference is CH3CCH, which peaks toward the c-C3H2 peak in L1521E, and toward the HNCO peak in L1544.

As seen in Sect. 4.1 c-C3H2 and CH3OH peak at different positions, similar to what was observed toward L1544 (Spezzano et al. 2016). However, the interpretation of the different spatial distributions is different for L1521E than for L1544. As it was shown by Spezzano et al. (2016) for L1544 (and unlike CH3OH as in Fig. 12), c-C3H2 probably originates in a layer-like structure as its intensities do not correlate with the H2 column densities, but rather show a flat distribution, tracing the external core envelope. However in the case of L1521E, even though they peak at different positions toward the core, c-C3H2 and CH3OH both show a correlation with the H2 column densities, which can be understood if the central density of the L1521E core is not high enough for catastrophic freeze-out to take place.

Overall, the chemical segregation in L1521E is not as developed as in L1544, which is probably related to the earlier evolutionary state of L1521E, and the fact that L1521E has not developed a high density central region, where freeze-out of C- and O-bearing bearing molecules dominates.

thumbnail Fig. 10

Column densities of different species as function of model predicted radius.

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6 Summary

We presented observations toward the starless core L1521E carried out with the IRAM-30 m telescope. The main results can be summarized as follows:

  • The lower limit on the CO depletion factor toward the dust peak of L1521E was found to be 4.3 ± 1.6, which is about one third of the value measured toward the more evolved L1544 core.

  • c-C3H2 and CH3OH peak atdifferent positions in both L1521E and L1544 (based on Spezzano et al. 2016), but the origin of the distributions toward the two cores is different, as toward L1521E both molecules show an increasing trend with increasing N(H2) while toward L1544 only the CH3OH intensities increase with increasing N(H2), while c-C3H2 shows a flat intensity profile. This difference is due to the lower central density of L1521E compared to L1544 and the fact that the region of catastrophic CO freeze-out is smaller in L1521E compared to L1544.

  • Comparing the relative abundances toward L1521E and L1544 we find that most sulfur bearing species such as C2S, HCS+, C34S, C33S, and HCS+ are more abundant toward L1521E than toward L1544. This is confirmed by a chemical model that reproduces most column densities measured toward L1521E within an order of magnitude. This suggests that significant sulfur depletion is taking place during the dynamical evolution of dense cores.

thumbnail Fig. 11

Comparison of relative abundances of species observed toward dust peak of both L1521E and L1544. As explained in Sect. 4.1, the abundances were derived using the column densities for an excitation temperature of 4.5 K for L1521E and 5 K forL1544.

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thumbnail Fig. 12

Integrated intensities of c-C3H2 (green symbols) and CH3OH (red symbols) as function of H2 column densities toward L1521E and L1544. The data plotted for L1544 are from (Spezzano et al. 2016).

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Acknowledgements

The work by A.V. is supported by the Russian Science Foundation via the project 18-12-00351. M.T. acknowledges funding from project AYA2016-79 006-P.

Appendix A The observed lines and their parameters

Table A.1

Molecules and transitions detected toward L1521E.

References


All Tables

Table 1

Molecular column densities derived toward the dust peaks of L1521E and L1544.

Table 2

Best fit column densities derived for L1521E with chemical code described in Vasyunin et al. (2017) and agreement with measured values given in Fig. 11.

Table A.1

Molecules and transitions detected toward L1521E.

All Figures

thumbnail Fig. 1

Spatial distributions of molecules detected toward L1521E (black contours) overplotted on N(H2) map (colors) derived from Herschel/SPIRE. The black dot, triangle, and asterisk show the c-C3H2, CH3OH, and HNCO peaks, respectively. The black square shows the Herschel dust peak. The beam size overplotted on the SO map shows the beam size of the SPIRE data, for the other maps it is the beam size of the IRAM-30 m data. For SO, CS, 13CS, C2S, and C34S the contour levels start from 3σ rms in steps of 3σ rms with 3σ rms values of 0.13, 0.38, 0.06, 0.12, and 0.11 K km s−1, respectively. For OCS and C33S the contour levels start from 6σ rms in steps of 4σ rms with 3σ rms levels of 0.02 K km s−1 for both species. For 34SO, C3S, CC34S, H2CS, and HCS+ the contour levels start from 9σ rms in steps of 6σ rms with 3σ rms values of 0.01, 0.02, 0.01, 0.07, and 0.03 K km s−1, respectively.

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In the text
thumbnail Fig. 1

continued. For C17O, CN, H13CN, HCN, and HCO+ the contour levels start from 3-σ rms in steps of 3-σ rms with 3-σ rms values of 0.05, 0.05, 0.04, 0.17, and 0.34 K km s−1, respectively. For N2H+, HN13C, HNCO, HC3N, and HC18O+ the contour levels start from 6-σ rms in steps of 4-σ rms with 3-σ rms levels of 0.08, 0.08, 0.05, 0.12, and 0.02 K km s−1, respectively. For CH3CN and CH3CCH the contour levels start from 9-σ rms in steps of 6-σ rms with 3-σ rms values of 0.01 and 0.03 K km s−1, respectively.

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In the text
thumbnail Fig. 1

continued. For H2CCO and HCO the contour levels start from 6-σ rms in steps of 4-σ rms with 3-σ rms levels of 0.02 K km s−1 for both species. For CH3CHO, C2H, C4H, and C4H2 the contour levels start from 9-σ rms in steps of 6-σ rms with 3-σ rms values of 0.01, 0.07, 0.03, and 0.004 K km s−1, respectively.

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In the text
thumbnail Fig. 2

Spatial distribution of c-C3H2 20,2 − 11,1 (white contours) and CH3OH (E2) (black contours). These are shown as detected toward L1521E overplotted on the N(H2) map which was derived from Herschel/SPIRE (color map), showing the large-scale environment of L1521E (top) and the spatial structure of molecular line emission close to the center of the core (bottom). The two overplotted lines show the location of the cuts along which the line intensities shown in Fig. 4 were extracted. The contour levels start from 6σ rms in steps of 6σ rms, with 3σ rms levels of 0.07 and 0.06 K km s−1 for CH3OH and c-C3H2, respectively.

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In the text
thumbnail Fig. 3

Line profiles of selected molecules observed toward CH3OH peak, c-C3H2 peak, and Herschel dust peak. The transitions used for each molecule are marked in Table A.1.

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In the text
thumbnail Fig. 4

Left: intensity cuts of selected molecules extracted along line crossing the two C17O peaks (close to c-C3H2 and CH3OH peaks) for various species (cut 1 in Fig. 1). Right: intensity cuts extracted along a line perpendicular to other cut and crosses Herschel dust peak (cut 2 in Fig. 1). The intensity values (y-axis) are normalized to the maximum value for each species, and a constant has been added to compare the normalized intensity distributions of the different species. The vertical dotted lines show the locations of the C17O and N(H2) peaks.

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In the text
thumbnail Fig. 5

Structure of L1521E. Left: integrated intensity of CH3OH (black contours) overplotted on integrated intensity of C17O. The black dot shows the peak of the 1.2 mm emission. The black triangle shows the peak of the H2 column density derived from the Herschel/SPIRE data. The CH3OH contour levels start from 6σ rms in steps of 6σ rms, with a 3σ rms level of 0.07 K km s−1. Right: integrated intensity of N2H+ (black contours) and distribution of 1.2 mm emission (white contours) overplotted on integrated intensity of C17O. The black dot shows the peak of the 1.2 mm emission. The black triangle shows the peak of the H2 column density derived from the Herschel/SPIRE data. The N2H+ contour levels start from 6σ rms in steps of 4σ rms, with 3σ rms level of 0.08 K km s−1. For the 1.2 mm data the contours start from 6σ rms in steps of 3σ rms with a 3σ rms level of 5.38 mJy.

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In the text
thumbnail Fig. 6

Maps of first four principal components obtained by performing PCA. The percentages represent the amount of correlation that can be reproduced by the single principal component. The red triangle represents the CH3OH peak, the asterisk is the c-C3H2 peak, and the square the is HNCO peak.

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In the text
thumbnail Fig. 7

Contribution of each transition to first two PCs, obtained by performing PCA on the standardized data.

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In the text
thumbnail Fig. 8

Correlation wheels where the coordinates for each molecule are their contribution to each PC, obtained by performing the PCA on the standardised data.

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In the text
thumbnail Fig. 9

Map of CO depletion factors toward L1521E. These were calculated using H2 column densities based on the Herschel/SPIRE data with β = 1.5 (left panel) and based on the 1.2 mm map (right panel), the derived C17O abundances, and a reference value of X(C17O) = 5.4 × 10−8. The contours show the spatial distribution of methanol and start from 6σ rms noise level in steps of 6σ rms. The triangle shows the peak of the 1.2 mm emission and the asterisk symbol shows the Herschel dust peak. The depletion factor toward the Herschel dust peak is about 4.3 based on the Herschel N(H2) values and about 3.9 based on the N(H2) values calculated from the 1.2 mm data. The black circles show the beam sizes that were used to calculate the depletion factors.

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In the text
thumbnail Fig. 10

Column densities of different species as function of model predicted radius.

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In the text
thumbnail Fig. 11

Comparison of relative abundances of species observed toward dust peak of both L1521E and L1544. As explained in Sect. 4.1, the abundances were derived using the column densities for an excitation temperature of 4.5 K for L1521E and 5 K forL1544.

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In the text
thumbnail Fig. 12

Integrated intensities of c-C3H2 (green symbols) and CH3OH (red symbols) as function of H2 column densities toward L1521E and L1544. The data plotted for L1544 are from (Spezzano et al. 2016).

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In the text

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