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A&A
Volume 559, November 2013
Article Number A84
Number of page(s) 20
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201321122
Published online 20 November 2013

© ESO, 2013

1. Introduction

It is firmly established that circumstellar discs around young stars are sites of planet formation. During the first 10 Myr, the initial gas-rich disc first evolves into a transitional and then finally into a debris disc while dispersing its gas content. The knowledge of this dispersal process and its properties is a crucial part of the planet-formation puzzle, as the amount of gas present in a disc determines whether gas-rich planets can form. Therefore, three aspects need to be characterised well: the disc geometry, the solid state content, and the gas content. Both the geometry and the dust in Herbig Ae/Be (HAEBE) discs are well-studied (e.g., the review by Williams & Cieza 2011; Meeus et al. 2001; Dominik et al. 2003; and Benisty et al. 2010 for the geometry; Acke et al. 2010 for the polycyclic aromatic hydrocarbon (PAH); and Juhász et al. 2010 for the dust properties).

To understand the spatial distribution, chemistry and physical properties of the gas component it is necessary to observe a range of transitions in several species, as they can originate in different regions of the disc under distinct conditions (density, temperature, and radiation field). For the purpose of studying the gas disc, H2 and CO lines are most often used, since they are the most abundant molecules present. In the FUV, H2 probes the warm gas and is frequently detected in T Tauri stars (TTS; e.g., Herczeg et al. 2006; Ingleby et al. 2011; France et al. 2012). In the IR, the detection of H2 is more difficult, due to the nature of the H2 molecule (homonuclear with wide spacing of energy levels). In a survey of 15 HAEBEs with CRIRES, Carmona et al. (2011) have detected ro-vibrational transitions at 2.1218 μm in only two objects. Earlier, Bitner et al. (2008), Carmona et al. (2008), and Martin-Zaïdi et al. (2009; 2010) searched for pure rotational lines of H2 at 17.035 μm in a sample of 20 HAEBEs; only two detections were reported. In TTS, the success rate is a little higher: Lahuis et al. (2007) have detected mid-IR H2 emission lines in six TTS, while Bary et al. (2008) have found five more H2 detections in the near-IR.

In sharp contrast, the detection of CO, although less abundant, is much easier. Its lines have been used to trace both the inner and outer disc. The fundamental ro-vibrational CO band (Δv = 1) at 4.7 μm traces warm (T > 100 K) gas in the terrestrial planet-forming region (0.1–10 AU). This band is often observed in HAEBEs (e.g., Blake & Boogert 2004; Brittain et al. 2007) and in TTS (e.g., Najita et al. 2003; Salyk et al. 2011). The bands are rotationally excited up to high J (>30), resulting in a Trot between 900 and 2500 K. The rotational temperature is much lower for 13CO at ~250 K (Brown et al. 2013). UV fluorescence can cause super-thermal level populations, as observed (Brittain et al. 2007) and modelled (Thi et al. 2013) in UV-bright HAEBEs, where Tvib> 5000 K, and in TTS (Bast et al. 2011; Brown et al. 2013). While some HAEBEs have CO extending to the dust sublimation radius (Salyk et al. 2011), others have evidence of significant inner disc gas clearing (Goto et al. 2006; Brittain et al. 2007; Pontoppidan et al. 2008; van der Plas et al. 2009). Thi et al. (2013), and van der Plas (2010) find that Trot<Tvib in flaring discs and the inner radius of CO is rin ~ 10 AU, while TvibTrot in self-shadowed discs, and rin ~ 1 AU: flaring HAEBE discs thus have lower CO abundances in their inner regions than flat discs. In a recent paper, Maaskant et al. (2013) show that several group I sources (the flaring discs) have dust-depleted gaps separating an optically thin inner disc and a flaring outer disc. It has been proposed that dust-depleted regions are a typical property of group I discs. This geometry allows more radiation to reach the outer disc, so they can be heated more when compared to the group II (flat discs). For the TTS, Brown et al. (2013) have found that most of the line profiles in TTS indicate that the CO emission originates in the disc with a slow disc wind.

On the other end of the spectrum, CO low-J pure rotational transitions (Δv = 0) can be observed at millimeter wavelengths. These low-J transitions originate in cold, optically thick CO that is located in the outer disc and are routinely detected in HAEBEs (e.g., Thi et al. 2001; Dent et al. 2005) and in TTS (e.g., Koerner & Sargent 1995; Öberg et al. 2010). As the lines are optically thick, the outer disc radius can be estimated from the line profile (e.g., Dent et al. 2005; Panić et al. 2008). In HAEBE discs, the dust temperature in the entire disc is high enough (at least 30 K) to avoid freeze-out of CO on the dust grains (e.g., Panic et al. 2009; Bruderer et al. 2012), while this happens more frequently in TTS (e.g., Hersant et al. 2009).

The much less observed far-IR wavelength range covers rotationally-excited mid- to high-J CO lines, which probe intermediate temperatures between the low-J sub-mm and the vibrationally-excited near-IR lines. Giannini et al. (1999) analysed ISO/LWS observations of 3 HAEBEs, covering rotational lines from Jup = 14 to Jup = 19. Their modelling showed that the CO emission originates in a compact region (up to a few 100 AU) around the central star and suggest that the stellar FUV radiation excites the CO.

The far-IR lines of CO in HAEBEs and TTS were also observed by DIGIT (Sturm et al. 2010) and GASPS (Meeus et al. 2012) with the instrument PACS (Poglitsch et al. 2010) onboard Herschel (Pilbratt et al. 2010). Bruderer et al. (2012; hereafter BR12) have modelled pure rotational CO transitions in HD 100546 (Sturm et al. 2010). They have showed the following: 1) On the disc surface, the gas temperature is decoupled from the dust temperature. 2) The low-J (J < 10) lines trace a radius between 70–220 AU at a low scale-height in the disc. The mid-J (J < 20) lines trace a radius of 35–80 AU, while the high-J (J > 20) lines trace the upper disc atmosphere at 20–50 AU. Therefore, the higher the J, the closer the star is to the region that the transition traces. 3) The highest J lines are optically thin and scale with the amount of hot gas in the disc surface, while the lowest J lines are optically thick and scale with the outer disc radius. The authors also show that the rotational diagram of HD 100546 (including also APEX data of low-J transitions) can be fitted with three temperature components: Trot ~ 60, 300, and 750 K.

We now present PACS spectra of a much larger sample of HAEBEs and TTS that was observed within the DIGIT Open Time Key Project (OTKP; P.I. Neal Evans). These spectra show pure rotational transitions of CO with Jup from 14 to 49, corresponding to Eup between 581 and 6724 K. Detections of O i, C ii, OH, H2O, and CH+ are reported in a companion paper by Fedele et al. (2013; Paper I). In Sect. 2, we present the sample and the observations, while we show the line detections and fluxes and construct rotational diagrams in Sect. 3. In Sect. 4, we look into the UV luminosity and use a small disc model grid to explain our observations. In Sect. 5, we speculate on the impact of PAH luminosity and inner disc clearing on the CO emission, while we present our conclusions in Sect. 6.

2. Sample and observations

Our sample consists of 22 HAEBEs and eight TTS. The HAEBE sample has spectral types between B9 and F4 and is representative for the HAe stars, while the TTS sources tend to be distinct from typical TTS in some way (such as having a large amount of veiling, or the presence of a strong disc wind, outflow, or a disc gap). We refer to Paper I for a discussion of the sample properties.

For each HAEBE star, we derived its disc group (I: flared discs, II: flat discs, in the classification of Meeus et al. 2001) and calculated LUV by integrating the UV flux obtained from the IUE archive between 1150 and 2430 Å for the HAEBEs. For the TTS, we measured LUV between 1250–1700 Å directly for a few sources. When direct measurements of FUV were not available, they were derived from observations of the H i Pfβ line and from an empirical relationship between Lacc and Pfβ (Salyk et al. 2013) in conjunction with an empirical relationship between Lacc and LUV (Yang et al. 2012). These results are listed in Table 1.

Table 1

Some properties of the sample.

We obtained Herschel/PACS spectroscopy in SED range mode, covering the spectral range between 51 and 210 μm. Not all sources were observed in the full range. For an overview of the observational settings and the reduction methods, we refer to Paper I, where we also show the full range spectra for each object. In Fig. 1 we zoom in at the positions of the CO lines for HD 100546, while we show similar plots in Figs. A.1 to A.8 for those objects where CO was detected.

With the sensitivity of our spectra, we are able to achieve a typical signal-to-noise ratio (S/N) on the continuum of 100 and to detect a line with 3σ confidence when the line flux is higher than 1× 10-17 W m-2, which varies with the observed wavelength and level of continuum flux. Furthermore, the spectra become more noisy at the edges of the bands, resulting in higher upper limits (for a more detailed discussion on this topic, we refer to the discussion in the DIGIT data paper by Green et al. 2013). For detected lines, we extracted the line fluxes using a Gaussian fit to the emission lines by using the rms on the continuum (excluding the line) to derive a 1σ error on the line flux. For non-detections, we give a 3σ upper limit, which is also calculated from the continuum rms. The derived line fluxes are listed in Table 2. The second row for each source is the flux density in the adjacent continuum.

thumbnail Fig. 1

Spectra of HD 100546, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

Table 2

Observed CO line fluxes in W m-2 × 10-18 (first row) and adjacent continuum flux density in Jy (second row).

3. Results

3.1. Detections

We detected CO lines in only 5 HAEBE stars of our sample: AB Aur, HD 36112, HD 97048, HD 100546, and IRS 48. While all 5 detections toward HAEBEs occur for fairly nearby stars ⟨ d ⟩  = 159 ± 71 pc, there are many HAEBEs at similar distances without detections. All 5 HAEBEs with detections have group I (flaring) discs; there is no CO detection seen in a group II (flat) HAEBE disc. Within the group I sources, the sources with CO detections have the highest LUV observed.

For the TTS, we detected CO in 4 objects: AS 205, a disc wind source (Bast et al. 2011) and a possible outflow source (Mundt 1984); S CrA and B, which are disc wind sources (Bast et al. 2011); RU Lup, which is a disc wind source (Bast et al. 2011) and an outflow source (Herczeg et al. 2005, and references therein); and DG Tau, which is an outflow source (e.g., Lavalley-Fouquet et al. 2000). An equivalent disc geometry classification for the TTS does not exist, so we could not compare the two samples in this regard. However, CO detections are only seen in TTS that show evidence of a strong disc wind (according to the line shape criterion defined in Bast et al. 2011), a resolved outflow, or both. It is not yet known whether there is a correlation between the presence of disc winds and outflows, although several discs have signatures of both.

The highest observable transition is J = 49 → 48, but the highest detected transition in our spectra is J = 36 → 35, as seen only in HD 100546 and in the outflow source DG Tau. In Table A.1, we list the transitions, their wavelength and the upper level energy (in kelvin) that are detected in our spectra. The transition J = 31 → 30 is at the same wavelength as an OH line; therefore, we consider it to be an upper limit in our analysis.

3.2. Rotation diagrams

Since we covered a wide range in transitions, it is possible to analyse our data with the aid of rotational diagrams (see Goldsmith & Langer 1999). We constructed rotational diagrams for the sources with at least three CO line detections (see Fig. 2) and list the derived Trot in Table 3. We derived the error on Trot by calculating the 2 extreme temperatures that are still consistent with the errors on the line fluxes. We define the Y axis of the diagram as ln  (dΩ  Nu/gu) = ln  (4π  Fu/(Auhν  gu)) to include the solid angle Ω of the emission, since the CO ladder could emerge from different regions in the disc (see discussion in BR12). For the HAEBEs, we obtain Trot between 200 and 350 K. For HD 100546, we also need a second component to fit the rotational diagram with Trot ~ 900–1000 K. A second component could also be present in AB Aur and HD 97048 but we do not have enough line detections to derive a temperature, in these cases. More sensitive data could reveal higher J emission lines also for these sources. The result for HD 100546 is similar to that found in an earlier analysis (Sturm et al. 2010). We note that the fit predicts line fluxes that are too high when compared to the observations for a few transitions (around 22 → 21, or Eup= 1397 K). It is not clear whether to attribute this to a bad flux calibration of this wavelength range (~118 μm) and/or to the low quality of this spectral region. The mean of the HAEBE Trot values, excluding the hot component in HD 100546, is ⟨ Trot ⟩  = 271 ± 39 K. For TTS, the temperatures are higher, ranging between 350 and 600 K with the outflow source DG Tau having the highest Trot at a value of 582 ± 12 K. The mean value is 486 ± 104 K, which is significantly higher than the mean for the HAEBEs.

The rotational temperatures of 200–350 K in HAEBE discs are lower than those typically found from 12CO ro-vibrational fundamental (v = 1 → 0) near-IR transitions (Salyk et al. 2011). They are comparable, however, to the rotational temperatures derived from the 13CO ro-vibrational fundamental lines, which probe distances somewhat further out into the disc (Brown et al. 2013).

We note that converting Trot derived from a rotation diagram to a kinetic temperature is not straightforward, as it assumes optically thin emission. Even if the lines are thin, Trot equals Tgas only if the excitation is in thermal equilibrium with the gas, i.e., if the density is high enough. Indeed, Bruderer et al. (2012) discussed that the CO ladder reflects emission from the disc atmosphere from a range of radii (~20–80 AU) at different temperatures and that the shape of the CO ladder can depend on several factors: 1) the gas temperature and density; 2) optical depth effects for the low-J to mid-J lines; and 3) either the amount of emitting molecules if the line is optically thin, or the size of the emitting region if the line is optically thick. For HD 100546, Bruderer et al. (2012) predicted that lines up to J = 16 → 15 are optically thick. Assuming that the model for HD 100546 is relevant for all our sources, the lines we observe are mainly optically thin so that the gas temperature and density are the main factors, assuming a normal CO abundance.

thumbnail Fig. 2

Rotational diagrams of the sources with at least 3 CO emission line detections (marked with a red asterisk, upper limits with an arrow). We also show a fit to the ladder, from which the rotational temperature can be derived.

4. Analysis

4.1. Relation with the (UV) luminosity

Does the far-IR CO line emission depend on stellar luminosities? We compared average HAEBE and TTS CO line luminosities for the J = 17 → 16 transition and found that they are remarkably similar: we find (2.06 ± 0.7) × 10-5L⊙ for the HAEBEs and (2.06 ± 1.0) × 10-5L⊙ for the TTS, while the average bolometric luminosities are 86.0 L⊙ and 5.0 L⊙ for the HAEBEs and TTS, respectively. This indicates that there is no correlation between the average line luminosity and Lbol.

But what about the UV luminosity? Models suggest that UV radiation is important for the gas heating and the excitation of the molecules and is one of the main factors determining the strength of the emission lines observed in the far-IR (e.g., Woitke et al. 2010; Kamp et al. 2011). Also, BR12 showed the importance of the temperature of the stellar radiation field with respect to the amount of line flux (see their Fig. 13). Because of self-shielding, the CO molecule can survive the UV radiation in the surface layers, and the CO emission lines can trace the heated disc surface. Since CO is an important coolant, an increased amount of heating leads to the detection of higher J lines, and a higher line luminosity.

The HAEBE stars with strong LUV and flaring discs (group I) often have a high [O i] 63 μm line flux (Meeus et al. 2012; Paper I). Furthermore, sources with higher [O i] 63 fluxes also have higher far-IR CO fluxes, as is shown in Meeus et al. (2012), for the CO J = 18 → 17 transition.

In HAEBEs, the bulk of the UV radiation is stellar, and the relative contribution from accretion is small. In contrast, the accretion luminosity is the most important contributor to the UV luminosity of TTS (e.g., Yang et al. 2012), as the relatively cool stellar photospheres produce very little UV flux. In Sect. 3, we showed that Trot is higher in our sample of TTS than in the HAEBEs. This can perhaps be attributed to a difference in the gas heating mechanism, which is stellar in HAEBEs, while it is due to accretion and possibly to shocks in the TTS. Podio et al. (2012) and Karska et al. (2013) show that heating by shocks is necessary to explain the observed molecular line emission in outflow sources. In our TTS sample, at least two sources with CO detections (DG Tau and RU Lup) show evidence of a jet.

Given the differences in origin of the UV radiation, which are important in the context of gas heating mechanisms, we discuss HAEBEs and TTS separately in the following sections.

Table 3

Trot derived from the rotational diagrams.

4.1.1. HAEBEs

The five HAEBEs with CO detections all have flaring discs with a hot disc atmosphere. Four out of five detections are in stars with a relatively high LUV, but there are also non-detections at LUV values that are similar to those of the detections, such as the transitional disc HD 141569 A. Unfortunately, the modest ranges in LUV and line luminosity coupled with the small sample size make it impossible to statistically test whether the line luminosity correlates with LUV.

4.1.2. TTS

Salyk et al. (2013) show that the accretion rate can also be correlated with the strength of the H Pfβ (n = 7 → 4) line at 4.65 μm, which was observed in all of the TTS in our sample. In the following, we use data from that paper, taken with either VLT/CRIRES (Pontoppidan et al. 2011) or Keck/NIRSPEC. In Fig. 3, we plot the H Pfβ line emission both for sources that do show the PACS CO lines and those that do not. All the sources with CO detections have strong H Pfβ lines, while most of the sources without CO, except for RNO 90, have weak H Pfβ lines. The ro-vibrational CO lines also appear to be stronger in the sources with PACS detections, especially for the v = 2 → 1 R(8) line at 4.6598 μm, which is enhanced by UV radiative excitation (Brown et al. 2013). This implies a connection between accretion rate and CO detections. This is perhaps due to the UV radiation provided by the accretion column. However, a direct connection with the value of Lacc is not found. We see a similar spread in Lacc for the detections and the non-detections, so that there is no clear correlation between these variables.

thumbnail Fig. 3

Observations of H Pfβ for the TTS sample; the line position is indicated with the vertical dashed line. Left: sources for which we have CO detections; right: sources for which CO was not detected with PACS.

Table 4

Parameters employed in the thermo-chemical models

thumbnail Fig. 4

Model gas density (ngas, top panels) gas-temperature (Tgas, center panels) and CO abundance (nCO/ngas, bottom panels) of thermo-chemical models with Mdisc = 10-2M⊙ and different flaring parameter ψ = 0.0, 0.1 or 0.2 (left to right columns).

thumbnail Fig. 5

Model CO ladder for the models with different gas mass Mdisc and flaring parameter ψ. The lines with Jup ≤ 7 are convolved to the APEX beam at the appropriate frequency; the higher Jup are convolved to PACS. Black arrows indicate the typical PACS detection limit, extracted from the AB Aur spectrum.

4.2. Thermo-chemical models of the mid- to high-J CO emission.

To explain the trend that far-IR CO lines are only detected towards flared discs, we ran a grid of thermo-chemical models introduced by Bruderer et al. (2012). These models solve for the dust radiative transfer, chemistry, and thermal balance in a self-consistent way. Details of the model and benchmark tests are reported in BR12. In that paper, the mid- to high-J CO emission of HD 100546 is reproduced with the fine structure lines of [O i] for a model tailored specifically to that disc. Here, we run a grid of generic disc models using a parameterised density structure. The surface density profile Σ(r) of the disc varies as (1)while the vertical density structure ρ(r,z) is a Gaussian (2)The scale-height H(r) is given by the power-law (3)The parameters assumed in the grid are given in Table 4. We vary the disc mass Mdisc and the flaring parameter ψ to study the dependence of the CO lines on these two parameters.

As a word of caution, we note that thermo-chemical modelling is prone to large uncertainties, particularly in the gas temperature calculation (Röllig et al. 2007). We thus refrain from fitting individual objects but aim to reproduce the main trends found in the observations.

How does the flaring parameter affect the temperature structure? In Fig. 4, the density, gas-temperature, and CO abundance profile of models with varying flaring parameters. ψ = 0.0,0.1 and 0.2, and Mdisc = 10-2M⊙, are shown. The figures are plotted such that the axes are the distance-normalised height (z/r) versus log (r). In the model without flaring (ψ = 0.0), there is always an attenuating column of gas and dust between the star and regions with density ≳106 cm-3. This density corresponds to the critical density of the CO J = 16 − 15 transition (Eup = 750 K), which is needed to excite the line collisionally. The mass of gas, which has a temperature high enough (T > 750 K) to excite the PACS CO lines, MCOex, is always small compared to the total disc mass (<0.5%). However, more gas can be irradiated and heated directly by the star to temperatures high enough for the excitation of the high-J CO lines for larger flaring parameters. For ψ = 0.0, MCOex = 4.2 × 10-6M⊙; for ψ = 0.1, MCOex = 2.0 × 10-5M⊙; and for ψ = 0.2, MCOex = 4.5 × 10-5M⊙: an increase up to a factor 10 with increasing flaring parameter ψ. Furthermore, the radial extent of the warm region also increases with amount of flaring. This is of importance, since lines up to J = 16–15 can be optically thick so that the size of the emitting area determines the amount of line flux (see BR12).

The abundance of CO (Figs. 4g, h, and i) is dominated by photodissociation in the upper atmosphere and the inner rim of the disc. The “warm-finger” of CO is produced by CO formation through the reaction of C+ with H2 to CH+, which only proceeds at high gas temperature (Jonkheid et al. 2007; Agúndez et al. 2008). Starting from CH+, a chain of reactions through CO+ and HCO+ then leads to CO. The species CH+ is detected in both HD 97048 and HD 100546 (Thi et al. 2011; Paper I). The CO lines discussed in this section form at large heights in the disc atmosphere and are not affected by any chemistry lower in the disc.

The CO ladder derived from the grid of models is shown in Fig. 5. Lines with Jup ≤ 7 are convolved to the beam of the APEX telescope, which has been used to detect these lines towards Herbig discs (e.g., Panić et al. 2010). Higher Jup lines are convolved to the beam of the PACS instrument. Typical PACS detection limits are shown by the example of AB Aur. We find that the flux in the PACS lines indeed depends considerably on the flaring of the disc. This is a direct result of the increasing mass of warm gas at several 100 K with the flaring parameter ψ.

On the other hand, the shape of the CO ladder does not vary significantly with disc mass, as long as Mdisc ≥ 10-3M⊙. While the CO lines above Jup ~ 16 are optically thin, the equilibrium of gas heating and cooling processes yields similar amounts of gas warm enough to excite the CO lines. This result is different from the findings of BR12, because they leave the dust structure unchanged and vary the gas-to-dust ratio. In the models presented here, we vary both gas and dust mass and keep the gas-to-dust ratio constant. It is the amount of dust which absorbs the impinging stellar light that is key for the heating of the gas.

From this grid of thermo-chemical models we conclude that the structure of the disc, and in particular the flaring, which allows direct stellar irradiation of a larger volume of gas, is key for the understanding of the CO ladder observed by PACS. Non-flared discs produce line emission flux that is too low to be detected in our spectra.

5. Discussion

What other parameters can influence the strength of the CO emission? In the following, we speculate about the influence of the PAH luminosity and the inner disc clearing on the far-IR CO emission. While PAHs are frequently detected in HAEBEs (up to 70%; Acke et al. 2010), they are seen in up to 15% of T Tauri discs (Geers et al 2006; Oliveira et al. 2007, 2010); therefore, we concentrate our discussion on PAHs observed in HAEBEs.

5.1. PAH luminosity

When present, PAHs contribute significantly to disc heating, absorbing UV photons in the upper disc layer. Afterward, they heat the disc through the photoelectric effect; as such, they also have an influence on the observed line strengths (e.g., Kamp & Dullemond 2004; Bruderer et al. 2012). Therefore, we searched for a link between the PAH luminosity and the observed CO emission. If we consider the sources with the highest LUV (>4 L⊙; AB Aur, HD 97048, HD 100546 – group I, and HD 141569 A, HD 150193 and HD 163296 – group II), we see that it contains equal amounts of sources with flaring and non-flaring discs, but the group II sources lack CO far-IR emission. As the amount of LUV does not solely determine the strength of the CO lines, we now also consider the PAH emission. In Table 1, we list LPAH for those objects where PAH bands are detected, while we plot the LPAH as a function of LUV for both sources with and without CO detections in Fig. 6. The group II sources HD 150193 and HD 163296 do not show PAH emission features, while the transitional disc HD 141569 A only has weak PAH bands when compared to the group I sources: LPAH  ~ 0.007 L⊙ versus 0.030 to 0.39 L⊙ for the 5 HAEBEs with CO detections. This suggests that strong PAH emission, in addition to strong UV luminosity, enhances CO far-IR emission, which is consistent with the non-detection in HD 141569 A. However, the PAH luminosity does not uniquely determine the CO line flux. It is interesting in this context to mention studies of the ro-vibrational bands of CO at 4.7 μm, revealing that HD 97048, HD 100546, and HD 141569 A have UV fluorescence, so that Tvib> 5600 K, while their Trot is distinct: it is ~1000 K for the first two and ~250 K for HD 141569 A (van der Plas 2010; Brittain et al. 2007), again indicating that the PAH luminosity is an important factor in the context of gas heating. However, BR12 showed that a change in the PAH abundance in their model of HD 100546 changes the high-J flux by less than a dex, leaving flaring as the most important parameter determining CO line fluxes.

thumbnail Fig. 6

LPAH as a function of LUV for the HAEBE sample. The red asterisks indicate sources where CO was detected in our spectra; the black diamonds are the sources with non-detections.

5.2. Inner-disc clearing

Based on a small sample of group I sources with spatially resolved mid-IR images, Maaskant et al. (2013) suggest that group I sources have dust-depleted regions in the inner disc (≲50 AU) that let the radiation penetrate further out in the disc. As a result, the outer regions, most notably the inner edge of the outer disc, experience more heating causing an excess at mid- to far-IR wavelengths. On the other hand, no flat (group II) discs have reported gaps in the literature. This suggests that these discs are relatively more self-shadowed and consequently cooler in the outer parts of the disc. In our HAEBE sample with CO detections, HD 97048, HD 100546, IRS 48, HD 36112 and AB Aur all have dust-depleted inner regions (Maaskant et al. 2013 for HD 97048; Bouwman et. al. 2003 for HD 100546; Geers et al. 2007; Brown et al. 2012; and van der Marel 2013 for IRS 48; Isella et al. 2010 for HD 36112; Honda et al. 2010 for AB Aur). This indeed suggests that dust clearing in the inner disc can result in more efficient heating of the outer layers, so that the amount of warm CO is increased.

5.3. Summary

Besides dust and gas temperature decoupling in the surface layers (due to flaring) and high UV luminosity, the discussion above suggests that the CO line luminosity in HAEBEs can also be enhanced by a combination of 1) PAH heating and 2) enhanced UV penetration of the outer disc due to a dust-depleted inner region. We expect these factors to also be inter-linked, but a detailed parameter study of disc models would be needed to explain the causality.

6. Conclusions

We analysed PACS spectra covering 53.5 to 190 μm to study the far-IR CO ladder in a sample of HAEBE and TTS. Our results can be summarised as follows:

  • 1.

    Out of 22 HAEBEs and 8 TTS, we detected CO emission lines in only 5 HAEBEs (23%) and 4 TTS (50%).

  • 2.

    In HAEBEs, the far-IR CO lines are only detected in flaring discs, indicating that the disc geometry (presence of a hot disc atmosphere) is important in creating detectable line emission. TTS with detections have evidence of a disc wind and/or outflow.

  • 3.

    The highest J transition (J = 36 → 35 with Eup = 3669 K) was observed in HD 100546 and the outflow source, DG Tau.

  • 4.

    We constructed rotational diagrams for the objects with at least 3 CO detections and derived Trot between 200 and 500 K. The mean values of the warm components are ⟨ Trot ⟩  = 271 ± 39 K for the HAEBEs and ⟨ Trot ⟩  = 486 ± 104 K for the TTS. The HAEBE HD 100546 is the only object that clearly needs a second, hot component of ~900 K, but other objects might also reveal a hot component when observed with a more sensitive instrument.

  • 5.

    We analysed the observed line fluxes with a small grid of thermo-chemical models. We found that an increased amount of flaring increased the CO line flux; a certain amount of flaring is necessary for the lines to become detectable with PACS and can increase the line flux by a factor 10. This is a direct result of the increase in warm gas mass with increasing amount of flaring when Tgas and Tdust are decoupled. Furthermore, we found that the disc mass is not a sensitive parameter, as long as it is higher than a few 10-4M⊙.

  • 6.

    For the HAEBEs amongst the group I sources, the ones with highest LUV have detectable CO lines.

  • 7.

    In TTS, sources with strong H Pfβ lines (a tracer of accretion rate) tend to have CO emission possibly because of the UV heating provided by the accretion column. However, a quantitative connection between Lacc and CO detections is not yet clear.

  • 8.

    We suggest that both strong PAH luminosity and the presence of a dust-depleted inner disc region can increase disc heating and by consequence, enhance the far-IR CO line luminosities.

Online material

Appendix A: Detection of CO transitions

In Table A.1, we list the transitions that were detected in our PACS spectra, while we show windows around the position of the CO lines for those objects where CO line emission was detected in Figs. A.1 to A.8.

Table A.1

CO Transitions detected in the PACS SED scans.

thumbnail Fig. A.1

Spectra of AB Aur, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.2

Spectra of HD 36112, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.3

Spectra of HD 97048, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.4

Spectra of IRS 48, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.5

Spectra of AS 205, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.6

Spectra of S CrA, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.7

Spectra of DG Tau, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

thumbnail Fig. A.8

Spectra of RU Lup, centered on the CO lines, indicated in red. In blue, we indicate the position of OH lines.

Appendix B: Upper limits

In Tables B.1 and B.2, we give the upper limits at a few key transitions (J36 → 35, 29 → 28, 23 → 22, 18 → 17, and 15 → 14) for those sources where not a single CO emission line was detected.

Table B.1

CO upper limits at selected wavelengths for the sources with full SEDs scans (50–200 μm) in W/m2 × 10-18.

Table B.2

CO upper limits for the sources with the forsterite scans (60–71 μm + 120–143 μm) in W/m2 × 10-18.

Acknowledgments

We would like to thank the PACS instrument team for their dedicated support. G. Meeus is supported by Ramon y Cajal grant RYC-2011-07920. B. Montesinos is partly supported by AYA-2011-26202. Support for this work, part of the Herschel “Open Time Key Project” Program, was provided by NASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain). This paper is based (in part) on observations made with ESO Telescopes at the Paranal Observatory under program ID 179.C-0151. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

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All Tables

Table 1

Some properties of the sample.

In the text
Table 2

Observed CO line fluxes in W m-2 × 10-18 (first row) and adjacent continuum flux density in Jy (second row).

In the text
Table 3

Trot derived from the rotational diagrams.

In the text
Table 4

Parameters employed in the thermo-chemical models

In the text
Table A.1

CO Transitions detected in the PACS SED scans.

In the text
Table B.1

CO upper limits at selected wavelengths for the sources with full SEDs scans (50–200 μm) in W/m2 × 10-18.

In the text
Table B.2

CO upper limits for the sources with the forsterite scans (60–71 μm + 120–143 μm) in W/m2 × 10-18.

In the text

All Figures

thumbnail Fig. 1

Spectra of HD 100546, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

In the text
thumbnail Fig. 2

Rotational diagrams of the sources with at least 3 CO emission line detections (marked with a red asterisk, upper limits with an arrow). We also show a fit to the ladder, from which the rotational temperature can be derived.

In the text
thumbnail Fig. 3

Observations of H Pfβ for the TTS sample; the line position is indicated with the vertical dashed line. Left: sources for which we have CO detections; right: sources for which CO was not detected with PACS.

In the text
thumbnail Fig. 4

Model gas density (ngas, top panels) gas-temperature (Tgas, center panels) and CO abundance (nCO/ngas, bottom panels) of thermo-chemical models with Mdisc = 10-2M⊙ and different flaring parameter ψ = 0.0, 0.1 or 0.2 (left to right columns).
In the text
thumbnail Fig. 5

Model CO ladder for the models with different gas mass Mdisc and flaring parameter ψ. The lines with Jup ≤ 7 are convolved to the APEX beam at the appropriate frequency; the higher Jup are convolved to PACS. Black arrows indicate the typical PACS detection limit, extracted from the AB Aur spectrum.
In the text
thumbnail Fig. 6

LPAH as a function of LUV for the HAEBE sample. The red asterisks indicate sources where CO was detected in our spectra; the black diamonds are the sources with non-detections.
In the text
thumbnail Fig. A.1

Spectra of AB Aur, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).

In the text
thumbnail Fig. A.2

Spectra of HD 36112, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.3

Spectra of HD 97048, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.4

Spectra of IRS 48, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.5

Spectra of AS 205, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.6

Spectra of S CrA, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.7

Spectra of DG Tau, centered on the CO lines, indicated in red. The OH line at 84.42 μm blends with the CO J = 31 → 30 line. In blue, we indicate the positions of other lines (CH+ at 90 μm and [O i] at 145 μm; rest are OH doublets).
In the text
thumbnail Fig. A.8

Spectra of RU Lup, centered on the CO lines, indicated in red. In blue, we indicate the position of OH lines.
In the text

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