© ESO, 2014

1. Introduction

Massive stars play a major role in the evolution of galaxies. From their birth in dense molecular clouds to their death as a supernova explosion, massive stars interact heavily with their surroundings by emitting strong stellar winds and by creating heavy elements (Zinnecker & Yorke 2007). They influence the formation of nearby low-mass stars and planets (Bally et al. 2005) and the physical, chemical, and morphological structure of galaxies (e.g. Kennicutt & Evans 2012). Although massive stars are an important component of galaxies, their formation processes are still unclear. It is difficult to observe high-mass star forming regions because of high dust extinction and their large distances and rapid evolution (Tan et al. 2014).

High-mass star forming regions are quite rare, so each observational effort is very helpful in solving they represent. One of the goals of the Herschel Space Observatory (Pilbratt et al. 2010) was to improve our understanding of the high-mass star formation processes. Among the several key projects devoted to those studies, we focus here on the Herschel key program Chemical Herschel Survey of Star Forming Regions (CHESS, Ceccarelli et al. 2010). The aim of this project is to study the chemical composition of dense regions of the interstellar medium and to understand the chemical evolution of star forming regions and the differences between regions with different masses/luminosities. The target sources of CHESS are the pre-stellar cores I16293E and L1544, the outflow shock spot L1157-B1, the low-mass protostar IRAS16293-2422, the intermediate-mass protostar OMC2-FIR 4, the intermediate-luminosity hot cores NGC 6334I and AFGL 2591, and the high-luminosity hot core W51e1/e2. Almost the entire spectral range of the HIFI instrument, that is, 480 to 1910 GHz, has been used to observe these objects. In this paper we focus on the source AFGL 2591.

Spectral surveys simultaneously cover a wide variety of molecular and atomic lines. In this way they offer the possibility to probe cold and warm gas and the fundamental processes that occur in star forming regions. Especially the wide frequency range of Herschel’s allowed us to cover molecular lines from very different energy levels, from light to heavier molecules, and thus to study the several species thoroughly.

AFGL 2591 is one of the CHESS sources. It is a relatively isolated high-mass protostellar object with a bipolar molecular outflow (Van der Tak et al. 1999). A massive sub-Keplerian disk has been proposed to exist around source AFGL 2591–VLA 3 (Wang et al. 2012). AFGL 2591 is located in the Cygnus X region, (. Based on VLBI parallax measurements of 22 GHz water maser, Rygl et al. (2012) have recently estimated the distance¹ towards AFGL 2591 to be 3.33 ± 0.11 kpc, hence, the corresponding luminosity is L = 2 × 10⁵ L_⊙ (Sanna et al. 2012). For a detailed source description see Van der Wiel et al. (2013, hereafter Paper I) and references therein.

The richness of the detected lines in AFGL 2591 from the HIFI/CHESS spectral survey gives us the opportunity to gain detailed insight into its chemical and physical structure. Results from the spectral survey will be presented in a series of papers. The first one focused on highly excited linear rotor molecules (Van der Wiel et al. 2013). In the present work the entire HIFI spectral survey of AFGL 2591 is presented.

Van der Wiel et al. (2013) studied linear rotor molecules (CO, HCO⁺, CS, HCN, HNC) in the high-mass protostellar envelope. This work was based on the Herschel/HIFI data together with observations from the ground-based telescopes, JCMT and IRAM 30 m. The line profiles of the observed emissions consist of two components, a narrow one that corresponds to the envelope, and a broad component from the outflow. The same nomenclature is used in the present paper.

This paper starts with the description of the observations and the data reduction of Herschel and JCMT spectra (Sect. 2). In Sect. 3 the general summary of the HIFI/CHESS spectral survey of AFGL 2591 is given. Here, all of the observed species from that survey are presented together with emission and absorption lines analysis. Discussions and conclusions are given in Sects. 5 and 6. Appendix A gives a table with all detected transitions and plots of their line profiles.

2. Observations and data reduction

2.1. 480−1850 GHz Herschel/HIFI data

Observations of AFGL 2591 (α₂₀₀₀ = ${\mathrm{20}}^{\mathrm{h}}{\mathrm{29}}^{\mathrm{m}}\mathrm{24}\stackrel{\mathrm{s}}{.}\mathrm{9}$, δ₂₀₀₀= +40°11′21″) were obtained with the Heterodyne Instrument for the Far-Infrared (HIFI, de Graauw et al. 2010) on board the ESA Herschel Space Observatory as a part of the HIFI/CHESS guaranteed time key programme².

A full spectral survey of AFGL 2591 of HIFI bands 1a−5a (480−1240 GHz, 18.4 h of observing time) was obtained. Nine additional selected frequencies were observed in 3.5 h of observing time. The corresponding bands are 5b (lines: HCl, CO), 6a (CO), 6b (CO), 7a (NH₃, CO) and 7b (CO, OH, [CII]).

Although this is the second in a series of papers based on HIFI/CHESS data of AFGL 2591 and a detailed description of its data reduction process has been provided in Paper I (Van der Wiel et al. 2013), basic information is recalled here as well.

The spectral scan observations were carried out using the dual beam-switch (DBS) mode, with the Wide Band Spectrometer (WBS) with a resolution of 1.1 MHz, corresponding to 0.66 km s^-1 at 500 GHz and 0.18 km s^-1 at 1850 GHz. The single-frequency settings were obtained in the dual beam-switch mode as well, with the fast chop and stability optimization options selected. Table 1 gives information about the covered frequency range, beam size, noise level, and integration time.

AFGL 2591 data were completely reduced with the with the Herschel interactive processing environment³ (HIPE; Ott 2010), version 8.1, using scripts written by the CHESS data reduction team (Kama et al. 2013). After pipelining, the quality of each spectrum was checked and spectral regions with spurious features (spurs) were flagged. Next, the correction for standing waves was made and a baseline was subtracted (polynomial of ~3). The final single-sideband spectrum is presented in Fig. 1.

Fig. 1 Complete baseline-subtracted spectrum. The strongest lines belong to CO and its isotopologues, while at 1901 GHz [CII] is seen.

Strong lines are known to create ghost features in the sideband deconvolution process (Comito & Schilke 2002). To check the importance of this effect on our data, the above steps were repeated with strong lines (especially CO transitions) masked out in the same way as spurs. The term strong lines refers to features of ${\mathit{T}}_{\mathit{A}}^{\mathrm{\ast }}\mathit{>}\mathrm{1}$ K in band 1a to >8 K in band 5a, depending on the amount of lines and the noise level in a given band. Following the outlined data reduction procedure, two single-sideband spectra for bands 1a−5a were obtained. The first set of spectra was used to analyse strong lines (e.g. CO and its isotopologues, HCO⁺), the second for line measurements of weak features, that is, those that were not masked as strong lines (e.g. SO, CH₃OH).

2.2. 330–373 GHz JCMT data

The excitation analysis of several molecules was complemented by ground-based observations from the James Clerk Maxwell Telescope (JCMT)⁴. These data are part of the JCMT Spectral Legacy Survey (SLS, Plume et al. 2007). The observations were taken with the 16-element Heterodyne Array Receiver Programme B (BHARP-B) and the Auto-Correlation Spectral Imaging System (ACSIS) correlator (Dent et al. 2000; Smith et al. 2008; Buckle et al. 2009).

The JCMT survey of AFGL 2591 covers the frequency range of 330−373 GHz with a spectral resolution of 1 MHz (~0.8  km s^-1). The beam size of the JCMT at these frequencies is 14−15′′, the image size is 2′. Detailed information about the data reduction and analysis can be found in Van der Wiel et al. (2011).

3. HIFI spectral survey of AFGL 2591

3.1. Detections and line profiles

Fig. 2 Average values of the line widths (top panel) and of the central velocity (bottom panel) from Gaussian fits for the observed emission lines of different molecules (”o” represents the outflow component). The emission lines of the envelope component are centred on −5.5 km s-1, as shown by the dashed line in the bottom panel.

From the Herschel/HIFI spectral survey, a total of 32 species (including isotopologues) were identified, resulting in 252 emission and 16 absorption lines (218 different transitions). Blended features were excluded from the analysis. Herschel surveys toward different sources revealed many spectral features that we currently cannot identify (e.g. Wang et al. 2011). However, no unidentified lines were found in our spectra.

For the line identification the JPL (Pickett et al. 1998) and CDMS (Müller et al. 2001, 2005) databases were used. Line analysis was made with the CASSIS software⁵. The presence of possible transitions resulting from an upper energy level E_up of less than 500 K was checked. Generally, detected lines have E_up< 400 K, except for the high-J CO transitions, which have E_up up to 752 K.

All the detected lines in the HIFI survey are presented in Table A.1, the entire spectrum is shown in Fig. 1, and corresponding line profiles can be found in Fig. A.1. For the sake of completeness all of the observed lines together with their profiles and measurements are presented in Table A.1, including the datasets of Paper I (Van der Wiel et al. 2013) and the complementary JCMT data.

Although the analysis of our survey revealed no new molecular species, some of our observed species have not been detected toward AFGL 2591 before. HIFI with its broad spectral range gave the opportunity to observe for the first time transitions of HF (Emprechtinger et al. 2012), OH⁺, CH, CH⁺ (Bruderer et al. 2010b) or C⁺ and HCl (this work) in AFGL 2591.

Within the object AFGL 2591, CH₃OH, SO₂, and SO show the highest number of detected transitions (54, 26, and 18 lines, respectively) among its identified species, followed by H₂CO and CO and its isotopologues. For the other molecules, a few lines at most were observed. The strongest transitions originate from CO and its isotopologues, HCO⁺, H₂O, and OH. In comparison, the remaining detected lines are relatively weak because of fluxes below 1 K km s^-1.

The lines were measured in the same way as described in Paper I. A Gaussian profile was fitted to each line, using the Levenberg-Marquardt fitter in the line analysis module of CASSIS. For most lines, a single Gaussian profile gave a good fit to the profile. However, for CO, ¹³CO, C¹⁸O, CI, [CII], HCO⁺, OH, and H₂O, double Gaussian profiles were needed to fit sufficiently narrow and broad line components. The measured parameters from Gaussian fits of the emission lines (central velocity and full width at half maximum) are plotted in Fig. 2 (together with the complementary JCMT data) as an average value for each molecule.

The narrow and single-line components are centred on −5.5 ± 0.5 km s^-1 (as derived before by Van der Tak et al. 1999) and originate from the protostellar envelope. Their line widths are of the order of 3.7 ± 0.9 km s^-1, whereas the broader line components (10.9 ± 4.2 km s^-1) are caused by the outflows and are centred on −6.3 ± 0.7 km s^-1. It was shown in Paper I that the outflow gas is not significantly different from that in the envelope, considering gas density, gas temperature, and the chemical balance of CO and HCO⁺.

3.2. Absorption line analysis

There are only a few absorption features observed toward AFGL 2591. A foreground cloud at V_lsr ~ 0 km s^-1 has been detected before, for example, Bruderer et al. (2010b), Emprechtinger et al. (2012), and Van der Wiel et al. (2013). In the CHESS/HIFI dataset we found 16 absorption lines; all measurements are listed together with emissions in Table A.1, and their lines profiles are presented in Fig. A.1. Most of them are red-shifted and associated with the foreground cloud at V_lsr ~ 0 km s^-1. Three broad, blue-shifted absorptions belong to the outflow lobe.

We derived the molecular column densities using the following relations:

$\begin{array}{ccc}& & {\mathit{N}}_{\mathrm{tot}}\mathrm{=}{\mathit{N}}_{\mathrm{l}}\frac{\mathit{Q}\mathrm{\left(}{\mathit{T}}_{\mathrm{ex}}\mathrm{\right)}}{{\mathrm{g}}_{\mathrm{l}}}\exp \left(\frac{{\mathit{E}}_{\mathrm{l}}}{\mathit{k}{\mathit{T}}_{\mathrm{ex}}}\right)\mathrm{\left[}\mathrm{c}{\mathrm{m}}^{-2}\mathrm{\right]}\mathrm{and}\\ & & {\mathit{N}}_{\mathrm{l}}\mathrm{=}\frac{\mathrm{8}\mathit{\pi }{\mathit{\nu }}^{\mathrm{3}}}{{\mathit{c}}^{\mathrm{3}}}\frac{{\mathit{g}}_{\mathrm{l}}}{{\mathit{g}}_{\mathrm{u}}{\mathit{A}}_{\mathrm{ul}}}\mathrm{\int }\mathit{\tau }\mathrm{·}\mathrm{d}\mathit{v}\mathrm{\left[}\mathrm{c}{\mathrm{m}}^{-2}\mathrm{\right]}\mathit{,}\end{array}$where Q(T_ex) is the partition function computed at the excitation temperature T_ex, ν is the frequency of the observed transition with the Einstein A-coefficient A_ul and the statistical weights of the lower g_l and upper levels g_u; c is the speed of light, and k is the Boltzmann constant. The line opacity τ was calculated from the measured brightness temperature T_mb and the temperature of the background continuum in a single side band T_c, using the relation  $\mathit{\tau }\mathrm{=}\mathrm{-}\ln {}^{\mathrm{\right(}}\frac{{\mathrm{T}}_{\mathrm{mb}}}{{\mathit{T}}_{\mathrm{c}}}^{\mathrm{\left)}}$.

N_tot and N_l are the total column density and the column density in the lower state of transition. The N_l may be the same as total column density for the ground state lines when the excitation temperature is very low (T_ex ~ 2.73 K). Therefore we applied Eq. (2) to calculate column densities for the ground-state transitions. For the absorptions that arise from the excited states we used Eq. (1) and assumed an excitation temperature of 10 K, as was derived for the foreground cloud in Paper I (see Table 4 in Van der Wiel et al. 2013).

The tentative absorption lines from a foreground cloud at V_lsr ~ 0 km s^-1 were observed of CCH (7₇−6₆ at 611.265 GHz), CH (two transitions: 3/2₂₊−1/2₁₋ at 532.724 and 3/2_{2 −}−1/2_{1 +} at 536.761 GHz), CH⁺ (1−0 at 835.138 GHz), H₂S (2₁₂−1₀₁ at 736.034 GHz), NH₃ (1₀−0₀ at 572.498 GHz), H₂O (two transitions: 1₁₀−1₀₁ at 556.936 and 1₁₁−0₀₀ at 1113.343 GHz), OH⁺ (three transitions: J = 2−1, F = 3/2−1/2 at 971.805, J = 1−1, F = 3/2−1/2 at 1033.004 and J = 1−1, F = 3/2−3/2 at 1033.119 GHz) and HF (1−0 at 1232.476 GHz). The estimated column densities for these species are listed in the upper part of Table 2.

Three broad absorptions are associated with the outflow (centred on ~−13.8 km s^-1): H₂O, CH⁺ and HF. Their column densities are presented in the lower part of Table 2.

Bruderer et al. (2010b) analysed hydrides toward AFGL 2591 using HIFI. Our column density results agree well, with their measurements within the errors: 3.1 × 10¹³ and 2.6 × 10¹³ cm^-2 for CH, 6.8 × 10¹³ and 1.8 × 10¹⁴ cm^-2 for the CH⁺ outflow component, 1.1 × 10¹⁴ and 1.2 × 10¹⁴ cm^-2 for CH⁺, and 3.0 × 10¹³ and 6.1 × 10¹³ cm^-2 for OH⁺, our results and from (Bruderer et al. 2010b) respectively. Bruderer et al. (2010b) also found lines of NH and H₂O⁺ in their spectra. These two species are not seen in our dataset, because of a slightly lower quality of spectral scans (Bruderer et al. 2010b have observations from the single frequency settings).

Based on Herschel data, Barlow et al. (2013) recently detected emission lines of  ³⁶ArH⁺ in the Crab nebula. Absorptions of this ion are also seen toward sources from Herschel Observations of EXtra-Ordinary Sources (HEXOS) and PRobing InterStellar Molecules with Absorption line Studies (PRISMAS) Herschel key programs (Schilke et al. 2014). The ³⁶ArH⁺J = 1−0 transition at 617.525 GHz is not detected in our spectra. The upper limit of the column density is 7.7 × 10¹² cm^-2 for the width of an absorption line of 1 km s^-1.

3.3. Emission line analysis

Fig. 3 Column densities and excitation temperatures estimated from the population diagrams, without the uncertain measurements, “o” represents the outflow component.

Fig. 4 Population diagrams. Open circles represent the observational data and crosses are the best-fit model from population diagram analysis. Dotted lines correspond to a linear fit to the rotational diagram.

To estimate column densities and excitation temperatures from the observed emissions we constructed rotational diagrams assuming that all lines for a given molecule have the same excitation temperature. This method is a useful tool for estimating the column densities and the excitation temperatures when many transitions of a particular species are observed. However, in many cases its accuracy is limited because it is based on the assumption that the emission lines are optically thin and the emissions fill the beam.

Goldsmith & Langer (1999) improved this excitation analysis method by introducing correction factors for the effects of the beam dilution and optical depth. Using this population diagram method, we estimated the column density, the excitation temperature, and the emission extent for each molecule with the observed multiple transitions. With three free parameters (column density, excitation temperature, and the beam filling factor), we used this method only when at least four lines for a given molecule were observed. Otherwise, only the rotational method was applied. The rotation diagram gives beam-averaged column densities, while the population diagram gives source-averaged values. Hereafter, all stated column densities (N_col) or excitation temperatures (T_ex) were derived from the population diagrams, except those of HNC and N₂H⁺, which were estimated from the rotational diagrams. At this point, the complementary JCMT data were crucial to increase the number of observed transitions for a given molecule.

The column densities of CO, HCN and HCO⁺ were obtained from their isotopologues (¹³CO, C¹⁸O, C¹⁷O, H¹³CN, HC¹⁵N, and H¹³CO⁺) using the standard isotopic ratios: ¹²C/¹³C = 60, ¹⁶O/¹⁸O = 500, ¹⁶O/¹⁷O = 2500, and ¹⁴N/¹⁵N = 270 (Wilson & Rood 1994).

All column densities and excitation temperatures values based on the rotational and population diagrams methods are given in Table 3. The opacities and emission sizes for each molecule derived from the population diagrams are listed in Table 3 as well. It also contains information about the covered energy E_up range for a given species and the number of lines from different energy levels that were used for the analysis. The values of the excitation temperatures and column densities are plotted in Fig. 3, excluding the uncertain measurements (i.e. HNC, N₂H⁺, CN, and NO).

Based on the optical depths values from Table 3, the lines of CN, CS, NO, and H₂CO can be characterised as optically thin (τ< 0.6). However, the results of CN and NO are uncertain because of only a few lines were observed. For optically thin lines calculations based on the rotational diagrams resulted in good approximations of the column densities and the excitation temperatures. The other molecular lines were characterised as optically thick. For these molecular species the population diagram method was more accurate.

The emission extent of the analysed molecules associated with AFGL 2591 ranges from around 2′′ (species such as SO, SO₂, and CH₃OH) up to 23′′ (CN). For most species emission sizes are smaller than 17′′.

From the comparison of the temperatures derived from the population diagrams (see the bottom panel of Fig. 3) it is possible to distinguish warm (e.g. CH₃OH and SO₂) and cold (e.g. HCN, H₂S, and NH₃) species. As cold species we classify those with excitation temperatures of up to 70 K. Warm molecules have higher temperatures, up to 175 K for SO₂. It is difficult to give an accurate borderline here and classify all species, but the wide range of excitation temperatures seems significant. Moreover, it was shown before by Bisschop et al. (2007) that some of the complex organic species can be classified as both warm and cold, which may indicate that they are present in multiple physical components.

The population diagrams are presented in Fig. 4. They show evidence for an excitation gradient of several species (HCO⁺, HCN, CS, and SO), which means that the population diagram method may be not enough to analyse all observed molecules. This is a motivation to use more sophisticated method in the near future (i.e., radiative transfer modelling) to study our spectral survey.

4. Discussion

4.1. CI and CII

C and C⁺ are the only atomic species found in our HIFI spectral survey of AFGL 2591. Both fine-structure transitions of neutral carbon, ³P${}_{\mathrm{1}}\mathrm{-}^{\mathrm{3}}$P₀ at 492 GHz and ³P${}_{\mathrm{2}}\mathrm{-}^{\mathrm{3}}$P₁ at 809 GHz, were observed towards AFGL 2591. These transitions consist of two components originating from the envelope and the outflow, similar to the CO lines (see Fig. A.1). CI was observed previously in AFGL 2591 by Van der Tak et al. (1999), but [CII] was observed for the first time with Herschel. The [CII] ²P${}_{\mathrm{3}\mathit{/}\mathrm{2}}\mathrm{-}^{\mathrm{2}}$P_1/2 line, an important interstellar coolant, shows several velocity components, two of them correspond to those in CI and CO. The [CII] line profile is distorted by a contamination from the off-position even after applying corrections within HIPE (Fig. A.1).

4.2. CO and its isotopologues

CO is one of the most often studied molecules (e.g. Mitchell et al. 1989; Black et al. 1990; Hasegawa & Mitchell 1995). Based on CO observation, Lada et al. (1984) found an extended bipolar outflow associated with AFGL 2591. Many strong lines of CO and its isotopologues (¹³CO, C¹⁸O, C¹⁷O) were also detected in our HIFI spectra, clearly showing the envelope and outflow components. The C¹⁷O lines are weaker and show only the envelope components. The abundance of CO = 3 × 10^-5 was calculated in Paper I. The CO column density in this work was estimated at 1.2 × 10¹⁹ cm^-2; Van der Tak et al. (2000b) derived a similar value of 3.4 × 10¹⁹ cm^-2.

4.3. HCO⁺

HCO⁺ was identified by intense lines in the HIFI and JCMT spectra. Moreover, three lines of H¹³CO⁺ were also positively detected. The abundance of HCO⁺ was estimated at 9 × 10^-9 (Paper I) and column density at 1.0 × 10¹⁴ cm^-2. Carr et al. (1995) estimated an abundance of 4 × 10^-10 and Van der Tak et al. (1999) derived [HCO⁺] = 1 × 10^-8 by using a model with lower H₂ column density.

4.4. N-bearing species

Six N-bearing species were observed in the HIFI spectra: HCN, HNC, CN, NO, N₂H⁺, and NH₃. All of these molecules have been detected before in AFGL 2591 (e.g. Takano et al. 1986; Carr et al. 1995; Boonman et al. 2001). Lines of N-bearing species observed with the Herschel/HIFI are weaker than those of CO and were fitted with a single-Gaussian profile, revealing these species to be components of the protostellar envelope, centred on −5.5 km s^-1. Only o-NH₃ shows a tentative absorption feature from a foreground cloud at V_lsr = 0 km s^-1. Two features observed with the JCMT, HCN 4-3, and HNC 4−3 show a contribution from the outflow and double-Gaussian profiles were fitted to these lines. We did not find NH and NH₂, which were detected in other HIFI spectral surveys (e.g. Zernickel et al. 2012). Upper limits are 0.8 K km s^-1 for the NH 1−0 line near 946 GHz  and 0.6 K km s^-1 for the NH₂ 1−0 line near 953 GHz. Upper limits were measured in the same way as in Paper I, that is, considering 3 km s^-1 a typical line width, hence using 5σ_rms × 3km s^-1. Among the observed features, two lines of vibrationally excited HCN 4−3, υ = 1c and υ = 1d are found (JCMT data). Line υ = 1c was observed before by Van der Tak et al. (1999). Boonman et al. (2001) analysed excited HCN, the 4−3 and 9−8 transitions. The interferometric observations from Veach et al. (2013) showed vibrationally excited υ = 1 and also υ=2 HCN 4−3 lines. These authors suggested that the υ = 2 HCN lines might be a useful tool to study a protostellar disk. Takano et al. (1986) observed ammonia transitions (1, 1) and (2, 2) with the Effelsberg 100 m telescope. They found a compact NH₃ cloud of around 0.6 pc diameter around the central source. These authors estimated a column density of 8 × 10¹³cm^-2. In comparison, calculations of our work gave a column density of 4.8 × 10¹³cm^-2.

4.5. S-bearing species

Of the S-bearing molecules we detected with HIFI CS, H₂S, H₂³⁴S, SO, and SO₂. All of these molecules have been observed before in AFGL 2591 (e.g. Yamashita et al. 1987; Van der Tak et al. 2003; Bruderer et al. 2009). Additionally, from the JCMT dataset we have several lines of these molecules, and also isotopologues of CS, SO, and SO₂ (¹³CS and C³⁴S, ³⁴SO, ³⁴SO₂), as well as OCS and o-H₂CS. SO and SO₂ show many weak lines of the envelope component. SO₂ is an example of warm species with an excitation temperature of 175 K, whereas H₂S is classified as colder species with an excitation temperature of 26 K. CS and SO have similar excitation temperatures, 61 K and 64 K. Van der Tak et al. (2003) studied the sulphur chemistry in the envelopes of massive star forming regions and found excitation temperatures of 185 K for SO₂, which is a similar result to the one calculated in this work. However, the column density of SO₂ varies strongly, 5.2 × 10¹⁴ cm^-2Van der Tak et al. (2003), 5.4 × 10¹⁷ cm^-2 our work. Results of column density of CS also differ in one order of magnitude, 3 × 10¹³ cm^-2 and 4.9 × 10¹⁴ cm^-2 (Van der Tak et al. 2003) and our work, respectively. The population diagram method is a good first step for the spectral surveys analysis, but in some cases a more advanced method is needed. Especially when there are not enough observed transitions from the lower energy levels for a given molecule, for example SO or CS and the excitation gradient is visible (see Fig. 4). We are planning to use radiative transfer modelling and estimate molecular abundances in the near future.

4.6. CCH, CH, CH⁺, OH, and OH⁺

Our spectra also revealed lines from the protostellar envelope and foreground clouds that belong to CCH, CH, CH⁺, OH, and OH⁺. CCH and CH show three absorption lines at ~0 km s^-1, OH⁺ three absorptions at ~3.6 km s^-1. Using HIFI, Bruderer et al. (2010b,a) found lines of CH, CH⁺, NH, OH⁺, and H₂O⁺, while lines of NH⁺ and SH⁺ have not been detected. Bruderer et al. (2010b) concluded that absorption lines of NH, OH⁺ and H₂O⁺ originate from a foreground cloud and an outflow lobe, while the emission lines of CH and CH⁺ are connected with the protostellar envelope (compare Sect. 3.2).

4.7. Water

Water lines have also been detected in our spectra. We found four transitions of o-H₂O (1₁₀−1₀₁ at 557 GHz, 3₁₂−3₀₃ at 1097 GHz, 3₁₂−2₂₁ at 1153 GHz, and 3₂₁−3₁₂ at 1163 GHz) and four transitions of p-H₂O (2₁₁−2₀₂ at 752 GHz, 2₀₂−1₁₁ at 988 GHz, 1₁₁−0₀₀ at 1113 GHz, and 2₂₀−2₁₁ at 1229 GHz). They show different profiles, mostly the envelope and outflow components, but also some absorptions (see Fig. A.1). For the envelope component we estimated a column density of 2.4 × 10¹⁵ cm^-2, an excitation temperature of 38 K, and an emission extent of 9.1′′. The full analysis of water lines in AFGL 2591 as part of the Water In Star forming regions with Herschel (WISH) Project will be presented in the forthcoming paper of Choi et al. (2014).

4.8. HF

HF is the only detected fluorine-bearing species in AFGL 2591. Its 1−0 transition at 1233 GHz was observed and analysed by Emprechtinger et al. (2012). They calculated an HF column density of  2 × 10¹⁴ cm^-2 and  4 × 10¹³ cm^-2 for emission and absorption.

4.9. HCl

Thanks to HIFI, many chlorine-bearing molecules (e.g. HCl, H³⁷Cl, H₂Cl⁺, H₂³⁷Cl⁺) were observed in different environments, for example toward protostellar shocks (Codella et al. 2012), diffuse clouds (Monje et al. 2013), and star forming regions (Neufeld et al. 2012). HCl and H³⁷Cl are the only observed chlorine-bearing species in our HIFI spectra of AFGL 2591. Three hyperfine components of HCl from the energy level of E_up= 30 K and two from the higher state E_up= 90.1 K were detected. In agreement with Neufeld et al. (2012), neither lines of H₂Cl⁺ nor lines of H₂³⁷Cl⁺ toward AFGL 2591 were found.

4.10. Complex species

From the HIFI spectral survey we found only two molecules (methanol and formaldehyde) that belong to complex organics. Bisschop et al. (2007) showed before that AFGL 2591 is a line-poor source. These authors analysed complex organic molecules in massive young stellar objects and found only a few of them in AFGL 2591; all of the intensities of the observed lines were very low. Many weak CH₃OH and H₂CO lines were detected in our HIFI spectra. Their column densities and excitation temperatures are 1.5 × 10¹⁷ cm^-2 and 108 K for CH₃OH, and 9.9 × 10¹³ cm^-2 and 41 K for H₂CO. Van der Tak et al. (2000a) estimated 1.2 × 10¹⁵ cm^-2 and 163 K for CH₃OH, and 8.0 × 10¹³ cm^-2 and 89 K for H₂CO. From the rotational diagrams Bisschop et al. (2007) derived 4.7 × 10¹⁶ cm^-2 and 147 K for methanol. All of these results vary slightly, but also suggest that methanol represents warm species.

5. Conclusions

The main conclusions from our AFGL 2591 spectral survey are as follows:

• 1.

  In the Herschel/HIFI spectral survey ofAFGL 2591 we observed 268 lines (excludingblends) of a total of 32 species. No unidentified features werefound in the spectra. JCMT data supplemented the excitationanalysis of several species seen in emissions.

• 2.

  Among the observed 268 lines, 16 absorptions were detected. Most of them belong to the known foreground cloud at V_lsr~ 0 km s^-1. Three broad absorptions are associated with the outflow lobe. The estimated column densities agree well with previous work.

• 3.

  Based on the population diagram method, the column densities and excitation temperatures were estimated. Molecular column densities range from 6 × 10¹¹ to 1 × 10¹⁹ cm^-2 and excitation temperatures range from 19 to 175 K. We can distinguish between species of higher (e.g. CH₃OH, and SO₂) and lower (e.g. HCN, H₂S, and NH₃) excitation temperature.

• 4.

  The population diagram method is a very useful tool for spectral survey analysis, but it is far from being perfect. Several species (HCO⁺, HCN, CS, and SO) show evidence of an excitation gradient, which is a motivation to use a more sophisticated method (i.e., radiative transfer modelling) in the near future to study molecules observed in the protostellar envelope of AFGL 2591.

Online material

Appendix A: HIFI/CHESS spectral survey

Fig. A.1 Line profiles of all identified species in HIFI spectral survey of AFGL 2591. Molecules (e.g. HCl, CH) for which hyperfine components were detected are plotted all together and are centred on the velocity of the middle line. Lines with multiply profiles (e.g. CO – envelope and outflow components; HF – emission and absorptions) are also presented in one figure and are centered at the velocity of the envelope component; please compare with Table A.1.

Fig. A.1 continued.

Fig. A.1 continued.

Acknowledgments

We thank Matthijs van der Wiel for providing JCMT data and useful discussions. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiologa (CSIC-INTA). Sweden: Chalmers University of Technology − MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University − Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC.

References

All Tables

All Figures

	Fig. 1 Complete baseline-subtracted spectrum. The strongest lines belong to CO and its isotopologues, while at 1901 GHz [CII] is seen.
In the text

	Fig. 2 Average values of the line widths (top panel) and of the central velocity (bottom panel) from Gaussian fits for the observed emission lines of different molecules (”o” represents the outflow component). The emission lines of the envelope component are centred on −5.5 km s-1, as shown by the dashed line in the bottom panel.
In the text

	Fig. 3 Column densities and excitation temperatures estimated from the population diagrams, without the uncertain measurements, “o” represents the outflow component.
In the text

	Fig. 4 Population diagrams. Open circles represent the observational data and crosses are the best-fit model from population diagram analysis. Dotted lines correspond to a linear fit to the rotational diagram.
In the text

Fig. A.1 Line profiles of all identified species in HIFI spectral survey of AFGL 2591. Molecules (e.g. HCl, CH) for which hyperfine components were detected are plotted all together and are centred on the velocity of the middle line. Lines with multiply profiles (e.g. CO – envelope and outflow components; HF – emission and absorptions) are also presented in one figure and are centered at the velocity of the envelope component; please compare with Table A.1.

In the text

	Fig. A.1 continued.
In the text

	Fig. A.1 continued.
In the text