Free Access
Issue
A&A
Volume 631, November 2019
Article Number A69
Number of page(s) 10
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201935441
Published online 22 October 2019

© ESO 2019

1 Introduction

Protoplanetary discs have been studied for decades and thousands of exoplanets have been discovered and characterised in the last 20 yr. Nevertheless, the mechanism by which discs evolve and lead to the formation of such planets is unclear. In particular, disc parameters, such as their surface density distribution and their vertical structure, and even more fundamental properties such as their total mass and the gas-to-dust mass ratio, are poorly constrained by observations.

Large millimetre(mm)-sized grains located in the disc midplane, which account for the bulk of the dust mass, are usually traced by their thermal continuum emission at (sub-)mm wavelengths. However, significant assumptions on the dust opacity, on its temperature, and on the optical depth of the continuum emission have to be made in order to obtain the total dust mass (see e.g. Andrews et al. 2013; Testi et al. 2014; Pascucci et al. 2016; Tripathi et al. 2017; Ansdell et al. 2018).

The bulk of the gas is typically traced by the emission from the lesser abundant isotopologues of carbon monoxide (CO) through pure rotational lines, which originate in the warm molecular layer of the disc. Being the second most abundant molecule after molecular hydrogen (H2), and having a well known and relatively simple chemistry, CO is typically used to trace the bulk of the gas in discs. More precisely, 13CO and C18 O emission is less optically thick and best suited to retrieving information about the column densities and to derive the disc total mass (van Zadelhoff et al. 2001; Williams & Best 2014). Freeze-out and isotope-selective photodissociation, the main processes governing the abundance of CO in discs, are implemented with various levels of detail in different physical-chemical disc models (e.g. Visser et al. 2009; Williams & Best 2014; Miotello et al. 2014; Woitke et al. 2016) and properly considered in the data analysis. Additionally, a significant assumption regarding the disc volatile [C]/[H] ratio is needed to convert CO mass to H2 mass; typically the value for the diffuse ISM, [C]/[H] = 1.35 × 10−4, is used and presumed constant throughout the disc, which results in CO locking up the bulk of the volatile elemental carbon with respect to hydrogen.

Thanks to the advent of the Atacama Large Millimeter/ submillimeter Array (ALMA), large surveys of protoplanetary discs in different star-forming regions have been carried out to study thegas and dust components simultaneously (Ansdell et al. 2016, 2017; Barenfeld et al. 2016; Eisner et al. 2016; Pascucci et al. 2016). One result that is common to these surveys is that CO emission from discs is fainter than expected (Miotello et al. 2017; Long et al. 2017). This may be interpreted as a lack of gas due to fast disc dispersal, or as a lack of volatile carbon that leads to faint CO lines (Favre et al. 2013; Ansdell et al. 2016; Schwarz et al. 2016; Miotello et al. 2017). The second interpretation is in line with previous studies where HD-based mass measurements of a few bright discs (TW Hya, DM Tau and GM Aur) were found to be incompatible with much smaller CO-based disc masses that assume a normal ISM volatile carbon abundance (Bergin et al. 2013; Favre et al. 2013; McClure et al. 2016). In particular, it was shown that the TW Hya disc needs to be depleted in volatile carbon by one to two orders of magnitude in order to reproduce the HD and CO isotopologue line emission (Favre et al. 2013; Kama et al. 2016; Trapman et al. 2017). Furthermore, in order to reproduce hydrocarbon observations, and more precisely C2H emission, volatile oxygen also needs to be depleted and by a larger amount than carbon, in order to get to carbon-to-oxygen ratios ([C]/[O]) larger than one (Kastner et al. 2015; Kama et al. 2016; Bergin et al. 2016). Pre-ALMA observations of C2H were carried out by Dutrey et al. (1997) for DM Tau and GG Ta, by Henning et al. (2010) for MWC 480, LkCa 15, and DM Tau, and by Kastner et al. (2015) for TW Hya. ALMA C2H data have been published for TW Hya, DM Tau, IM Lup, and V4046 Sgr (Bergin et al. 2016; Cleeves et al. 2018; Kastner et al. 2018). Similarly to TW Hya and DM Tau, volatile carbon and oxygen abundances have been constrained in the IM Lup disc based on the detection of C2H and were found to be depleted (Cleeves et al. 2018). During revision of this manuscript, Bergner et al. (2019) published a survey of C2H observations for 14 discs observed with ALMA spanning a range of ages, stellar luminosities, and stellar masses, which cover some of the sources mentioned above.

More generally, it is known that volatile oxygen must be depleted thanks to the weak or non-detections of H2O lines observed by the Herschel Space Observatory (Hogerheijde et al. 2011; Du et al. 2017). This implies that volatile oxygen is locked up in larger bodies in the midplane that do not circulate to larger disc heights where the water would be photodesorbed. Carbon depletion may be caused by two different processes. Similarly to oxygen, carbon may be locked up in larger bodies and sequestered in the midplane (Krijt et al. 2018). Another option is chemical transformation of carbon, most notably into CO2, CH3OH, and hydrocarbons (Eistrup et al. 2016, 2018; Yu et al. 2017a,b; Bosman et al. 2017, 2018; Schwarz et al. 2018).

If [C]/[O] > 1, some carbon is left free from CO and is available to create more complex molecules, such as hydrocarbons. Moreover, if the process of volatile depletion happens by incorporating carbon and oxygen in larger grains which settle to the midplane and drift inwards, the disc regions where this happens would be more exposed to UV radiation. High carbon-to-oxygen ratios combined with strong UV irradiation are the right conditions to boost hydrocarbon emission (Bergin et al. 2016). If the faint CO lines found in protoplanetary discs are due to high levels of volatile depletion rather than to low gas-to-dust ratios, this should be reflected in strong hydrocarbon – for instance C2H – fluxes.

To address these questions, new Cycle 4 ALMA C2H line emission data were collected for a subsample of discs in the Lupus star-forming region and are presented in Sect. 2. The main results, shown in Sect. 3, are discussed in Sect. 4 where C2H line emission is linked to the volatile [C]/[O] ratio in discs.

2 Observations

Observations of ten protoplanetary discs in the Lupus star-forming region were acquired with ALMA in Cycle 4, under the project code 2016.1.00459.S (PI: Miotello A.). The targets were selected among the Lupus discs with the brightest continuum emission and that were detected in CO isotopologues and in CN emission (Ansdell et al. 2018; van Terwisga et al. 2019). Their CO-based gas-to-dust ratios range from 100 to 0.1 (see Table 1). Six sources, J16011549-4152351, J16070854-3914075, MY Lup, Sz 114, Sz 71 and Sz 98, were observed on 2017 July 12 with 49 antennas (16.7–2700 m baselines) using J1517-2422 and J1610-3958 as calibrators. Four targets, J16070384-3911113, J16090141-3925119, Sz 129 and Sz 68, were observed on 2017 July 15 with 42 antennas (18.6–1500 m baselines) using J1427-4206 and J1610-3958 as calibrators. The typical integration time was approximatively 20 min on-source. We estimate a flux calibration error of 10% based on variations in the flux calibrators.

The spectral setup included five windows covering C2H, c-C3H2, SO, H13 CO+, and H2CO lines, and one spectral window devoted to the continuum emission centred on 246 GHz. Here, together with the continuum data, we present only the C2H (N = 32, J = 7∕25∕2, F = 43 and F = 32) line observations, whose spectral window was centred on 262.006 GHz, with a bandwidth of 117 MHz, a channel width of 0.14 MHz, and a velocity resolution of 0.16 km s−1. Only two hyperfine components (F = 43 and F = 32) of the C2H (N = 32, J = 7∕25∕2) line were covered. No clear detection of the other molecules was found in any of the targeted sources.

From the pipeline calibrated data we performed (only phase and not amplitude) self-calibration using the continuum emission of each source. The data were imaged using the clean task in CASA version 4.7.2 using natural weighting and the achieved beam size is ~ 0.2 × 0.3 arcsec. Images are presented in Fig. 1. The typical achieved rms is between 5 and 10 mJy km s−1. Integrated fluxes with the achieved rms are reported for each source in Table 1, together with 13CO integrated fluxes and continuum flux densities taken from Ansdell et al. (2016).

Similarly to what was found in the 12CO data, the C2H emission in J16011549-4152351 is completely contaminated by the foreground cloud. Therefore, it is impossible to extract any meaningful information on the hydrocarbon emission of the disc. For this reason, J16011549-4152351 is excluded from the analysis presented inthis paper.

thumbnail Fig. 1

Moment-zero maps for C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 3–2) in the nine Lupus sources obtained integrating the emission over the channels where signal was detected. No Keplerian masking was used to produce such maps. White contours show the continuum emission at 3σ, 20σ, and at 20σ intervals.

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Table 1

Target list with integrated fluxes and distances.

3 Results

3.1 Integrated fluxes

Seven of the nine discs in our sample are detected in C2H emission, which is generally bright and of the same order as the 13CO emission (see Table 1). The moment-zero maps of the C2H line emission in our targets are presented in Fig. 1. These maps are obtained by integrating the emission from all channels where signal was detected. The disc integrated C2H fluxes (reported in Table 1) are measured on the image plane using an aperture synthesis method on the moment-zero maps, similar to that used by Ansdell et al. (2016). The line luminosity is obtained by multiplying the integrated flux by 4πd2, where d is the distance of the target in parsecs as measured by Gaia (see Table 1, Gaia Collaboration 2018). The C2H integrated line luminosity is plotted in Fig. 2 against 13CO over continuum at 890 μm fluxes, where the continuum total flux density is integrated over a band width of 1 km s−1 for convenience (Table 1, Ansdell et al. 2016). The 13CO-to-continuum flux ratio can be used as an observational proxy of the gas-to-dust mass ratio, under the assumption that CO is the main carbon carrier in the disc warm layer. The typical sensitivity calculated at 162 pc, the distance of Sz129 which is our lowest detection, is shown by the grey solid line. A bimodal distribution is observed: either C2H emission is strongly detected at more than a 5σ level, or such a molecule is not detected. A possible interpretation of this behaviour is discussed in Sect. 4. The C2H emission is likely optically thin for the observed sources in Lupus, as discussed below.

3.2 Ring-like structures

ALMA is revolutionising the field of planet-formation by showing that discs often present substructures, almost always in the shape of cavities (e.g. van der Marel et al. 2016), rings, and gaps (see Long et al. 2018; Andrews et al. 2018, and references therein). Such structures, seen in continuum emission, are possibly related to perturbations of the disc gas surface density distribution due to the presence of forming planets, or to different suites of instabilities (e.g. Johansen et al. 2009; Bai & Stone 2014; Flock et al. 2015). Rings have also been detected in molecular line emission, but are of a different nature (see e.g. Bergin et al. 2016; Cazzoletti et al. 2018; van Terwisga et al. 2019). The cause of such rings varies for different molecules, but is related to chemistry rather than a physical perturbation of the disc structure. Rings seen in DCO+ and N2H+ are, for example, possibly linked to the location ice rings, such as that caused by the CO snow-surface (e.g. Mathews et al. 2013; Öberg et al. 2015; van ’t Hoff et al. 2017). In other cases, such as for CN and for C2H, there is no relation with the snow lines (Bergin et al. 2016; Cazzoletti et al. 2018; van Terwisga et al. 2019). The link between the structures seen in the continuum and in different gas tracers is being investigated with current and upcoming observations.

Ring-like structures are detected both in continuum emission at 1.2 mm and in C2H line emission also in our sample. Thanks to the much higher sensitivity compared to the Ansdell et al. (2016) data, our ALMA observationsallowed us to detect continuum emission at 1.2 mm at high signal-to-noise ratios (S/Ns). The continuum maps of the nine discs presented here are shown by the white contours in Fig. 1. On the image plane, only Sz 98 shows the presence of a ring in continuum emission located at 90 au, as already hinted at by the modelling on the uv -plane by Tazzari et al. (2017) and by van Terwisga et al. (2019) (see Fig. 1). The sources Sz 71, Sz 114, and Sz 129 are also included in the ALMA large program “DSHARP” (Andrews et al. 2018) and, when observed at such high angular resolution, they show ring and gap structures in the outer discs (Huang et al. 2018).

As shown in Fig. 1, C2H emission in Sz 114 is clearly ring-shaped. The C2H azimuthally averaged intensity radial profile is computed after derotating and deprojecting the images following van Terwisga et al. (2019) (see blue line in Fig. 3). The azimuthally averaged continuum emission in our observations is shown by the pink line in Fig. 3. The peak of C2H emission is located at 40 and 27 au, respectively, in these two sources. The former peak coincides with the CN emission peak found by van Terwisga et al. (2019) in Sz 71. The location of the gaps and rings detected using very-high-angular-resolution ALMA data (~30 to 60 mas) by Huang et al. (2018) in these two sources is indicated by the dotted and solid pink vertical lines, respectively, while the peak of the C2H emission is shown by the blue verticalline. The C2H emission ring location does not seem to be connected with either the gaps or the rings seen in continuum emission in Sz 71. The angular resolution of our C2H observations is 30 au, and this allows us to rule out that the C2H ring location coincides with any of the rings and gaps discovered in the continuum by Huang et al. (2018). This suggests a chemical origin for the hydrocarbon rings, similar to what found for CN in the same location (Cazzoletti et al. 2018; van Terwisga et al. 2019). For Sz 114, the interpretation is less clear, as the distance between the C2H ring and thecontinuum substructures is smaller than our angular resolution. Proper chemical modelling to interpret the nature of the observed molecular rings is beyond the scope of this paper.

thumbnail Fig. 2

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) integrated line luminosity of nine Lupus protoplanetary discs vs. the logarithm of 13CO (J = 3– 2) integrated line flux over the continuum total flux density at 890 μm (integrated over a band width of 1 km s−1 for convenience). The solid grey line shows the typical sensitivity obtained at the distance of our lowest detection (Sz129, at 162 pc).

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4 Discussion

Following Bergin et al. (2016) and Kama et al. (2016), if depletion of volatile carbon and oxygen happens in such a way that [C]/[O] > 1 in the gas, discs with lower carbon abundance and fainter CO isotopologue lines would show brighter hydrocarbon lines. If [C]/[O] > 1, not all volatile carbon is locked in CO but some is available to start the hydrocarbon chemistry. Furthermore, if the process of volatile depletion happens by incorporating carbon and oxygen in larger grains which settle to the midplane and drift inwards, the disc regions where this happens would be more exposed to UV radiation. High carbon-to-oxygen ratios combined with strong UV irradiation are the right conditions to boost hydrocarbon emission (Bergin et al. 2016). Therefore, if the faint CO lines found in Lupus discs are due to high levels of volatile depletion rather than to low gas-to-dust ratios, this should be reflected in strong hydrocarbon emission. It may be expected that sources with volatile C and O depletion and thus faint CO lines will show bright C2H lines and vice versa.

thumbnail Fig. 3

Azimuthally averaged normalised brightness profiles of the C2H (blue) and 1.2 mm continuum (pink) emission as functions of disc radius for Sz 71 (top panel) and Sz 114 (bottom panel). The blue vertical lines show the peak location of the C2H emission, while the dotted and solid pink lines show respectively the location of the gaps and rings detected in these discs by Huang et al. (2018) with higher-angular-resolution ALMA data (DSHARP large program; Andrews et al. 2018).

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4.1 CO-based gas-to-dust ratios

The 13CO and C18O integrated fluxes and continuum flux densities from Ansdell et al. (2016) can be used to estimate CO-based gas and dust masses. Miotello et al. (2017) have employed the grid of physical-chemical models run with the code DALI (Bruderer et al. 2012) presented previously in Miotello et al. (2016), which include isotope-selective processes and freeze-out, to estimate CO-based gas masses for the discs in Lupus. Such models cover a realistic range of disc and stellar parameters, sample disc masses from 10−5M to 10−1M, and produce simulated spatially integrated continuum and line fluxes. For each mass bin, Miotello et al. (2017) computed a median integrated 13CO line luminosity and compared such values with observations (Ansdell et al. 2016) to determine the disc gas mass which is an input parameter in the DALI models. The uncertainty on the mass estimate was given by the dispersion from models with different disc parameters. The overall CO-based gas masses are very low, often lower than one MJ. As a consequence, the typical gas-to-dust mass ratios found in Lupus are much lower than the factor of 100 measured in the ISM (Miotello et al. 2017). Such low CO-based gas-to-dust ratios may in fact be an indication of volatile carbon and oxygen depletion, rather than fast gas dispersal in Lupus discs.

4.2 Dependence of C2H emission on CO-based gas-to-dust mass ratios

It is not presently clear how tight any anti-correlation between C2H and CO-based gas-to-dust ratios should be, and even if this should be linear. This is in fact probably not the case, as there are competing effects at play that may differ from source to source. If carbon is depleted by a factor of 100, one could naively explain why CO emission is found to be a factor of 100 fainter than expected, but not C2H emission, which should be fainter bya factor of 104. Instead, C2H is found to be bright in the studied discs and this may be due to volatile [C]/[O] > 1 which boosts the C2H emission irrespective of the low C abundance. How much carbon depletion and [C]/[O] > 1 balance each other and which one prevails depends on source details and amount of depletion, but a tight relation is not expected. Any non-correlation or anti-correlation goes already in the direction of the [C]/[O] > 1 hypothesis.

A simple interpretation of the bimodal distribution seen in Fig. 2 could be that the volatile [C]/[O] ratio is acting as an on/off switch of the C2H emission. The C2H lines are bright only if [C]/[O] > 1 and the hydrocarbon chemistry path is allowed; otherwise C2H lines would be much fainter, and even undetected in our specific case. This would be in line with chemical models showing that the C2H column densities are boosted as soon as the [C]/[O] becomes larger than unity (see e.g. Cleeves et al. 2018, and Sect. 4.5). Such a scenario would explain the sharp transition between detections and non-detections seen in Fig. 2. All detections in our sample are much brighter than the typical sensitivity of our observations, shown by the grey line in Fig. 2. Our interpretation is however limited by the reduced statistical significance due to small sample sizes: our sample may simply lack intermediate cases. Enlarging the sample of deep C2H observations in Lupus would allow us to better describe the relation between C2H- and CO-based gas-to-dust ratios and to learn more about the volatile depletion scenario.

4.3 C2H emission in other sources

Recently, Bergner et al. (2019) published C2H observations for a set of 14 discs spanning a range of ages, stellar luminosities, and stellar masses, including the previously observed discs by Dutrey et al. (1997), Henning et al. (2010), Guilloteau et al. (2016), Bergin et al. (2016), Cleeves et al. (2018), and Kastner et al. (2018). This sample is complementary to our set of observations, as we target fainter and less radially extended discs shown to be more representative of the disc bulk population (see e.g. Ansdell et al. 2016, 2018; Pascucci et al. 2016; Long et al. 2018), with similar stellar parameters and age (Lupus is 1–3 Myr old, Comerón 2008).

The Lupus discs are shown by the black symbols in Fig. 4, while the additional sources are reported in red. The C2H non-detections are shown by the empty triangles as 3-σ upper limits. The observations presented by Bergner et al. (2019) are of a lower sensitivity than ours, with 3-σ limits typically a factor ten higher, and they do not allow us to rule out the hypothesis of a step-like behaviour of the C2H emission by populating the plot with intermediate cases, that is, with the logarithm of C2H-integrated line luminosities between 6.8 and 7.5. The discs in the Bergner et al. (2019) sample that might populate this range of C2H luminosity are all upper limits. On the other hand, our step-like behaviour hypothesis is strengthened by their detections, that are all as bright as or brighter than our Lupus observations.

The two samples are however complementary. In Fig. 4, the dotted lines show the median integrated C2H line luminosity of the Lupus discs, in black, and of the additional sources, in red, which is 0.5 dex higher. The discs presented by Bergner et al. (2019) host in many cases more luminous stars (e.g. HD 163296, MWC 480), show brighter millimetre continuum fluxes, and are also much larger in their radial extent (see Tables 5 and 6 of Bergner et al. 2019). The UV field impinging onto the disc, which is intrinsically connected with the stellar properties, and the disc outer radius are two parameters that most likely affect the C2H emission, similarly to what was found for CN by Cazzoletti et al. (2018) and van Terwisga et al. (2019).

The question we would like to answer with this paper however, regards the nature of the faint CO isotopologue emission of the Lupus discs (Ansdell et al. 2016; Miotello et al. 2017). As found also in Chamaeleon, the bulk of the disc population seems to be composed of faint and compact discs (Pascucci et al. 2016; Long et al. 2017). The open question is whether such discs have low gas masses or low volatile carbon and oxygen abundance, as both scenarios would lead to faint CO fluxes. Discs surrounding brighter stars and with larger radial extent, such as those presented for example by Bergner et al. (2019), do not show fainter-than-expected CO emission (see e.g. HD 163296, Kama et al. 2016). A proper description of the C2H emission dependence on higher UV fields and larger disc radii is beyond the scope of this paper, despite it being an interesting and important topic to be explored with models, similarly to what was done for CN by Cazzoletti et al. (2018). For these reasons, we focus here on the Lupus disc observations and compare them with an explorative grid of models representative of such targets (see Sect. 4.5).

thumbnail Fig. 4

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) integrated line luminosity of the new Lupus observations presented in this paper in black (left panel) and of the discs presented by Bergner et al. (2019) in red (right panel). Empty triangles show the C2H non-detections as 3-σ upper limits.The dotted lines show the median integrated C2H line luminosity of the detected Lupus discs, in black, and of the detected additional sources, in red. We note the much larger dynamic range of our ALMA data.

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4.4 C2H and CN line luminosity in Lupus

Figure 5 shows integrated C2H line luminosity for the nine Lupus sources as a function of integrated CN line luminosity. The C2H non-detections are also reported and shown as upper limits at the 3-σ level by the empty triangles. We compute the linear regression coefficients using the fully Bayesian method by Kelly (2007), which allows the inclusion of upper limits, uncertainties on both axes, and intrinsic dispersion in the fitting procedure.

A correlation between the logarithm of C2H and CN line luminosity is found for the Lupus discs. The best representative set of parameters, which we adopt as the median of the results of the chains, is shown in Fig. 5. The intercept and the slope of the linear regression are α = − 12.6 ± 4 and β = 2.4 ± 0.4, respectively. The best representative model has a correlation coefficient of 0.96± 0.04. As already mentioned, this may be an additional indication that C2H and CN have a similar or common driver in protoplanetary discs, as found by Bergner et al. (2019) for C2H and HCN. The linear correlation found on the log-log space is however steeper than two, suggesting that C2H emission must depend more strongly on a particular parameter than CN emission. On the other hand, C2H and CN emission luminosities do not show different trends with stellar or accretion luminosity (see Figs. A.1A.4), ruling out a different dependence on the UV field.

Kastner et al. (2014) and Guilloteau et al. (2016) showed that C2H and CN abundances may be enhanced in more evolved discs. Following this suggestion, Sz 68 and J16070384-3911113, the two Lupus sources where C2H was not detected, may be the youngest discs in our sample. However, there is no indication from their classification that they are less evolved than the other seven sources. This may simply be a sign that Sz 68 and J16070384-3911113 are chemically less evolved. On the other hand, the survey by Bergner et al. (2019) was explicitly designed to check for evolutionary trends, as it contains discs with ages from roughly 1 to 13 Myr. The authors find that there is no clear relation between older discs and brighter C2H lines. The caveat to their analysis is that, due to optical depth effects, fluxes may not be reliably tracing molecular abundances. This problem should however not apply to our Lupus observations, as the observed fluxes are lower than those found by Bergner et al. (2019).

thumbnail Fig. 5

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. CN (N = 32) integrated line luminosity (van Terwisga et al. 2019) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits. The solid red line shows the best fit obtained using the Bayesian fitting procedure by Kelly (2007), while the light redlines show a subsample of results from some chains.

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4.5 Comparison with models

In order to better quantify the effect of carbon and oxygen depletion on the hydrocarbon line emission, a grid of physical-chemical disc models run with the code DALI (Dust and LInes; Bruderer et al. 2012) was employed. This is a similar set of models as presented by Cazzoletti et al. (2018) for CN. They are designed to cover the typical disc parameter space in terms of disc mass, vertical structure, and radial extent, with an updated chemistry described by Visser et al. (2018). More specifically, we chose T Tauri-like disc models, where the central star emission is described by a black body at Teff = 4000 K plus an UV excess which mimics a typical mass accretion rate, and Lbol = 1 L. The large-over-small grains mass fraction is flarge = 0.99 and large grains are settled toward the midplane with a settling paramer χ = 0.2 (see Cazzoletti et al. 2018 for more details). Such models assume typical ISM-like gas-phase elemental abundances, more precisely [C]/[H] = 1 × 10−4 and [O]/[H] = 3.5 × 10−4 ([C]/[O] ~ 0.3). A detailed source-by-source modelling is beyond the scope of this paper. We employ the aforementioned grid of models to obtain physically and chemically motivated predictions that we can compare with the observed C2H integrated line luminosities.

The simulated C2H integrated line luminosities are presented in Fig. 6 as functions of gas disc mass and are compared with the Lupus observations. In panel a, the model results obtained with ISM-like gas-phase elemental abundances are shown by the dotted black line, while the black symbols show the observed C2H integrated line luminosities versus CO-based gas masses. Our model results underestimate the detected C2H integrated line luminosities shown by the black circles by one to two orders of magnitude.

A subsample of models was run, reducing the carbon abundance by two orders of magnitude and the oxygen abundance by slightly larger factors, in order to reach volatile [C]/[O] = 0.3, 0.8, 1.0 and 1.5. The model results, presented in panel b of Fig. 6, are shown by the coloured squares (in orange, light green, dark green, and blue for [C]/[O] = 0.3, 0.8, 1.0 and 1.5, respectively, for Mdisc = 10−2M). Furthermore, a set of models with [C]/[O] = 1.5 was run for the whole mass range (Mdisc = 10−510−2M) and the simulated C2H integrated line luminosities are shown by the blue solid line. If the global volatile carbon and oxygen abundances are decreased by two orders of magnitude, but the [C]/[O] ratio is kept fixed to 0.3, the C2H integrated line luminosities are fainter by a factor of five (see orange square in comparison with the dotted black line). If however the volatile [C]/[O] ratio increases and overcomes the [C]/[O] = 1 threshold, C2H line luminosities are boosted by more than one order of magnitude (see comparison between orange and blue squares). Our Lupus C2H detections may be compatible with a volatile carbon depletion of two orders of magnitude, and [C]/[O] > 1, as shown in panel b where the observed data points are shifted toward larger disc masses by a factor of 100. If carbon is depleted by two orders of magnitude, CO isotopologue lines are also fainter by roughly the same factor and the actual disc masses are a factor of 100 larger (Miotello et al. 2016). This first-order comparison with model predictions suggests that volatiles may be strongly depleted and [C]/[O] > 1 in the Lupus discs. The two sources that were not detected in C2H, Sz 68, and J16070384-3911113 may be discs with genuine gas depletion. In fact, according to Fig. 6, they would need to have very low masses to be reproduced by any of the models.

While our C2H model results show that C2H line emission increases by more than one order of magnitude if the volatile [C]/[O] ratio increases from 0.3 to 1.5 (see Fig. 6), the CN line fluxes on the other hand are found to vary only by a factor of two (see Cazzoletti et al. 2018, Fig. 14). The stronger dependence of C2H lines on volatile [C]/[O] compared with CN lines may explain the steep correlation shown in Fig. 5 and tells us something about the similar but different origin of these two molecules.

Bergin et al. (2016) investigated the effects of the volatile [C]/[O] on the intensity of the C2H emission. The authors ran different sets of models with ISM-like volatile [C]/[O] = 0.4, and with enhanced [C]/[O] ratios equal to 1 and 2. With an ISM-like abundance of carbon and oxygen ([C]/[O] = 0.4) their simulated emission is well below observed values in DM Tau and TW Hya. With [C]/[O] = 1, the simulation comes closer to the data, but [C]/[O] > 1 is needed to approach the observed values, similarly to the Lupus discs. The same conclusion was found by Kama et al. (2016) for TW Hya. This shows that bright C2H observations do not simply imply a large volatile [C]/[O] ratio, but specifically that such a ratio is larger than unity. Similarly, an increase in the C2H simulated column densities by two orders of magnitude is found by Cleeves et al. (2018), as shown in their Fig. 4, going from [C]/[O] = 0.95 to 1.86. A similar behaviour is also found with our DALI models, where C2H column densities increase by two orders of magnitude if we increase [C]/[O] from 0.8 to 1.5 (see Fig. B.1).

In order to test the dependence of the C2H emission with an actual gas depletion, we ran the same set of standard models shown by the dotted black line in Fig. 6, but decreasing the gas masses by two orders of magnitude while keeping the same dust masses. This provides a set of models with gas-to-dust ratios equal to unity, the results of which are presented in Fig. 7 by the orange solid line. Our Lupus C2H observations are also reported as a function of their dust masses which were derived from their continuum emission (see Miotello et al. 2017). These models fail to reproduce the brighter observed C2H line luminosity; the difference between the observations and these models is even greater than comparisons with the models with a gas-to-dust ratio of 100. The fainter C2H detection, that is from the source Sz 129, could be consistent with models with a gas-to-dust ratio of 1 if we compare with the disc dust mass (Fig. 7); however this does not hold if we plot the model results against the total gas mass based on CO for ISM [C] and [O] (as in Fig. 6, left). Interestingly, J16070854-3914075 may be compatible with a genuine gas depletion. The source Sz 68 is a more complicated case, being part of a known binary system (Andrews et al. 2018).

thumbnail Fig. 6

Simulated C2H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc gas masses compared with the Lupus observations. Panel a: dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and [C]/[O] = 0.3. The Lupus detections are shown by the black circles and non-detections by the empty triangles as 3-σ upper limits. Panel b: solid blue line shows model results obtained assuming that [C] and [O] abundances are depleted by two orders of magnitude and [C]/[O] = 1.5. Orange, light green, green, and blue squares show the model predictions when elemental [C] and [O] are depleted by two orders of magnitude and volatile [C]/[O] = 0.3, 0.8, 1.0, and 1.5 for Mdisc = 10−2M. The black circles and empty triangles show C2H observationsas a function of CO-based gas masses multiplied by a factor of 100 to account for the carbon depletion.

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thumbnail Fig. 7

Simulated C2H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc dust mass compared with the Lupus observations. The dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and a gas-to-dust ratio of 100 (same as in panel a of Fig. 6). The solid orange line shows model results obtained decreasing the gas masses by two orders of magnitude, while keeping the dust mass fixed in order to obtain a gas-to-dust ratio of 1. Lupus detections are shown by the black circles, and non-detections by the empty triangles as 3-σ upper limits.

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5 Summary and conclusions

This work presents new ALMA observations of C2H together with higher signal-to-noise-ratio continuum data at 246 GHz for a subsample of nine protoplanetary discs in the Lupus star-forming region. Seven out of nine discs are detected in C2H emission. After performing self-calibration, the C2H integrated luminosities were compared with CO-based gas-to-dust ratios (Miotello et al. 2017). The seven detections have much brighter fluxes than the typical sensitivity of our data, hinting at a step-like distribution of the discs in our sample that results from the [C]/[O] ratio being larger or smaller than unity. This interpretation is supported by physical-chemical models able to reproduce the observations only if volatile carbon and oxygen are depleted and the [C]/[O] ratio is larger than unity. Models with reduced gas-to-dust ratios fail instead to reproduce our C2H observations. Our study is limited however by small sample sizes, but the conclusion is strengthened when literature data on other sources are included. Other effects such as UV luminosity and disc size may cause scatter from source to source.

A steeper-than-linear correlation is found between C2H and CN emission lines in our Lupus discs. This can be explained by the fact that C2H line fluxes depend more strongly on variations of the volatile [C]/[O] ratio than CN line fluxes. This implies that these two molecules have a similar but different origin in discs.

Ring-like structures are detected both in C2H emission and in continuum (Huang et al. 2018) but they do not seem to be connected. The source Sz 71 shows ring-shaped emission both in C2H and in CN and the location of the peak intensity coincides for the two tracers.

The results presented in this paper suggest that volatile carbon and oxygen depletion is at play in the Lupus discs, and is consistent with the 13CO and C2H observations. This however does not necessarily rule out fast gas dispersal (see e.g. J16070854-3914075) as an explanation for the low CO fluxes and CO-based gas masses, since actual gas disc masses remain difficult to quantify (Ansdell et al. 2016; Pascucci et al. 2016; Miotello et al. 2017; Long et al. 2017). This has important implications for disc evolution and planet-formation theories, as disc gas masses are possibly larger than what CO observations would suggest if CO is assumed to be the main carbon carrier in the gas phase.

Acknowledgements

The authors wish to thank Richard Booth, Antonella Natta, Karin Öberg, and Jennifer Bergner for insightful comments and interesting discussions. A.M. and S.F. acknowledge an ESO Fellowship. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823823, (RISE DUSTBUSTERS). This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Ref no. FOR 2634/1 ER685/11-1. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00459.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

Appendix A Dependence of C2H and CN emission on L and Lacc

The nine Lupus discs targeted in this paper have also been detected by van Terwisga et al. (2019) in CN (N = 4 − 3) emission. In this section we plot C2H and CN emission line luminosity as a function of stellar and accretion luminosity (Alcalá et al. 2017, see Table 1) in Figs. A.1A.4.

thumbnail Fig. A.1

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. stellar luminosity (Manara et al. 2018) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits.

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thumbnail Fig. A.2

Logarithm of CN (N = 32) vs. stellar luminosity (van Terwisga et al. 2019; Manara et al. 2018) for the Lupus discs.

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thumbnail Fig. A.3

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. accretion luminosity (Manara et al. 2018) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits.

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thumbnail Fig. A.4

Logarithm of CN (N = 32) vs. accretion luminosity (van Terwisga et al. 2019; Manara et al. 2018) for the Lupus discs.

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Appendix B C2H column densities

The simulated C2H column densities obtained with a subsample of our DALI models are shown in Fig. B.1. The total disc mass is fixed to 10−2M, volatile C abundance is reduced by two orders of magnitude (with respect to ISM abundance) and O abundance is decreasedby the same value or more in order to obtain [C]/[O] ratios of 0.3 (orange line), 0.8 (light green line), 1.0 (green line) and 1.5 (blue line). Similarly to what is found by Cleeves et al. (2018), the C2H column density increases by more than one order of magnitude if we increase [C]/[O] from 0.8 to 1.5 (see Fig. B.1).

thumbnail Fig. B.1

C2H column densities as a function of the disc radius obtained with DALI for a 10−2 M disc. Volatile C abundance is reduced by two orders of magnitude and O abundance is decreased by the same value or more in order to obtain [C]/[O] ratios of 0.3 (orange line), 0.8 (light green line), 1.0 (green line) and 1.5 (blue line).

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All Tables

Table 1

Target list with integrated fluxes and distances.

All Figures

thumbnail Fig. 1

Moment-zero maps for C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 3–2) in the nine Lupus sources obtained integrating the emission over the channels where signal was detected. No Keplerian masking was used to produce such maps. White contours show the continuum emission at 3σ, 20σ, and at 20σ intervals.

Open with DEXTER
In the text
thumbnail Fig. 2

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) integrated line luminosity of nine Lupus protoplanetary discs vs. the logarithm of 13CO (J = 3– 2) integrated line flux over the continuum total flux density at 890 μm (integrated over a band width of 1 km s−1 for convenience). The solid grey line shows the typical sensitivity obtained at the distance of our lowest detection (Sz129, at 162 pc).

Open with DEXTER
In the text
thumbnail Fig. 3

Azimuthally averaged normalised brightness profiles of the C2H (blue) and 1.2 mm continuum (pink) emission as functions of disc radius for Sz 71 (top panel) and Sz 114 (bottom panel). The blue vertical lines show the peak location of the C2H emission, while the dotted and solid pink lines show respectively the location of the gaps and rings detected in these discs by Huang et al. (2018) with higher-angular-resolution ALMA data (DSHARP large program; Andrews et al. 2018).

Open with DEXTER
In the text
thumbnail Fig. 4

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) integrated line luminosity of the new Lupus observations presented in this paper in black (left panel) and of the discs presented by Bergner et al. (2019) in red (right panel). Empty triangles show the C2H non-detections as 3-σ upper limits.The dotted lines show the median integrated C2H line luminosity of the detected Lupus discs, in black, and of the detected additional sources, in red. We note the much larger dynamic range of our ALMA data.

Open with DEXTER
In the text
thumbnail Fig. 5

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. CN (N = 32) integrated line luminosity (van Terwisga et al. 2019) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits. The solid red line shows the best fit obtained using the Bayesian fitting procedure by Kelly (2007), while the light redlines show a subsample of results from some chains.

Open with DEXTER
In the text
thumbnail Fig. 6

Simulated C2H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc gas masses compared with the Lupus observations. Panel a: dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and [C]/[O] = 0.3. The Lupus detections are shown by the black circles and non-detections by the empty triangles as 3-σ upper limits. Panel b: solid blue line shows model results obtained assuming that [C] and [O] abundances are depleted by two orders of magnitude and [C]/[O] = 1.5. Orange, light green, green, and blue squares show the model predictions when elemental [C] and [O] are depleted by two orders of magnitude and volatile [C]/[O] = 0.3, 0.8, 1.0, and 1.5 for Mdisc = 10−2M. The black circles and empty triangles show C2H observationsas a function of CO-based gas masses multiplied by a factor of 100 to account for the carbon depletion.

Open with DEXTER
In the text
thumbnail Fig. 7

Simulated C2H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc dust mass compared with the Lupus observations. The dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and a gas-to-dust ratio of 100 (same as in panel a of Fig. 6). The solid orange line shows model results obtained decreasing the gas masses by two orders of magnitude, while keeping the dust mass fixed in order to obtain a gas-to-dust ratio of 1. Lupus detections are shown by the black circles, and non-detections by the empty triangles as 3-σ upper limits.

Open with DEXTER
In the text
thumbnail Fig. A.1

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. stellar luminosity (Manara et al. 2018) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits.

Open with DEXTER
In the text
thumbnail Fig. A.2

Logarithm of CN (N = 32) vs. stellar luminosity (van Terwisga et al. 2019; Manara et al. 2018) for the Lupus discs.

Open with DEXTER
In the text
thumbnail Fig. A.3

Logarithm of C2H (N = 3 2, J = 7∕25∕2, F = 43 and F = 32) vs. accretion luminosity (Manara et al. 2018) for the Lupus discs. Empty triangles show the C2H non-detections as 3-σ upper limits.

Open with DEXTER
In the text
thumbnail Fig. A.4

Logarithm of CN (N = 32) vs. accretion luminosity (van Terwisga et al. 2019; Manara et al. 2018) for the Lupus discs.

Open with DEXTER
In the text
thumbnail Fig. B.1

C2H column densities as a function of the disc radius obtained with DALI for a 10−2 M disc. Volatile C abundance is reduced by two orders of magnitude and O abundance is decreased by the same value or more in order to obtain [C]/[O] ratios of 0.3 (orange line), 0.8 (light green line), 1.0 (green line) and 1.5 (blue line).

Open with DEXTER
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.