Time Variability of Molecular Line Emission in IRC+10216

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1 Time Variability of Molecular Line Emission in IRC David Teyssier, ESA-ESAC, Spain J. Cernicharo, G. Quintana-Lacaci, M. Agúndez, M. Barlow, E. de Beck, F. Daniel, L. Decin, P. GarciaLario, M. Groenewegen, D. Neufeld, J. Pearson

2 IRC IRC (CW Leo) is the nearest carbon AGB star (123 pc Groenewegen et al. 2012) Highly obscured at low wavelengths but uniquely bright in the FIR and millimetre lines, it has become the archetype star for study of mass-loss and envelope evolution on the AGB Presents a complex and multiple shell structure (see Homan s talk) Its circumstellar shell has revealed a remarquable chemical laboratory with more than 70 molecules discovered to date, (e.g. Cernicharo et al. 2000) Virtually observed at any available wavelength and spatial/spectral resolution FoV Decin et al See also posters S1-10, S1-14 for more recent results Blue: VLT FORS1 V-band (Leão et al. 2006), Green: PACS 70 µm, Red: PACS 100 µm

3 IRC as a Mira IRC+10216, like most luminous carbon stars, is strongly variable in the optical and NIR Fitted light-curves from various studies indicative of periods in the range days Most recent NIR fit places max. of light on JD = (23 Mar 2008) Menten et al FIR photometry monitoring gives similar period, with a certain time lag (Groenewegen et al. 2012) I(0.81) Alksnis et al P = 639 ± 4 d P = 630 ± 4 d Groenewegen et al SPIRE 250 µm Menten et al. 2012

4 Thermal line variability in IRC Only limited attempts to study line intensity variation over time Some lines known to be masering have significant modulation (e.g. H 2 O, OH esp. in the cm, SiS, SiO in the mm) Carlström et al. (1989) reported variation of two SiS v=0 rotational lines around 3 mm They however note the large impact of calibration uncertainty Cernicharo et al. (2000): no noticeable variability in their 2 mm survey within the 20% calibration accuracy (IRAM 30m) Overall this lead to the general assumption that most thermal lines in AGB are non-variable, and their use as secondary calibrators in many mm/submm facilities Carlström et al. 1989

5 Thermal line monitoring with Herschel (2) Project started almost by accident Overlapping spectral regions taken ½ yr apart in two projects revealed unexpectedly anomalous intensities in some lines Black: spectral survey (May 2010) Cernicharo et al. Red: Hydrides search (Dec 2010) Agúndez et al.

6 Thermal line monitoring with Herschel (1) A total of five dedicated observing slots were conducted with HIFI/SPIRE/PACS, typically separated by 6 months Combined to additional data (e.g. calibration) this leads to 7, 8 and 7 epochs resp. for HIFI/SPIRE and PACS

7 Analysis method Line and continuum signals are separated for each instrument Integrated line intensities computed for each available HIFI lines (~90), and selected ladders in SPIRE and PACS ( 12 CO, 13 CO, HCN v=0 and v 2, CS, H 2 O, etc) unresolved lines can suffer blending Light-curves are fitted as cosine with four free parameters (amplitude, mean level, period, time of maximum light) Intensity series within instrumental uncertainty considered flat

8 Examples SPIRE CS lines PACS HCN v=0 + v 2 =1 blend

9 Some highlights - continuum Continuum measured from the spectrometers shows clear modulation, similar to that of photometer (Groenewegen+2012) Periods typically in the range days Modulation amplitude (F max /F min ) tends to increase with freq. PACS SPIRE

10 Some highlights HCN vibrational modes HCN intensity variations observed in v=0, v 1 =1, v 2 =1 and v 2 =2 (overtone) and v 3 =1 states in several rotational levels Amplitude of the modulation tends to scale with energy levels and is particularly enhanced in the vibrationally excited states Maser! ν 1 =1: 3311 cm -1 (3 µm) ν 2 =1: 713 cm -1 (14 µm) ν 2 =2: 1426 cm -1 (7 µm) ν 3 =1: 2097 cm -1 (4.8 µm)

11 Some highlights IR pumping CCH shows the largest amplitude modulation of the line sample (N=7-6 and 8-7, in excess of 10) de Beck et al. (2012) showed that the inclusion of the first three vibrational modes was crucial to model the HIFI lines intensities This radiative pumping occurs in Infrared bands (3-27 µm), where the star flux already experiences large variations Interestingly the 1 st stretching mode v 1 is coupled to the 1 st electronically excited state (A 2 Π), lying at 2.5 µm, where the star flux could have its largest amplitude modulation de Beck et al. 2012

12 Some highlights CO variability (1) 12 CO and 13 CO are cases where a transition between a non-variable and variable regime takes place The switch occurs in a smooth transition layer that spreads over several energy levels no regime jump from one level to another From the HIFI perspective, J=10-9 appears flat while J=16-15 seems variable (no transition measured in-between with HIFI) From the SPIRE/PACS perspective, transitions above J=13-12 could be already modulated but calibration uncertainties are similar to modulation amplitude PACS lines from J=28 onwards (no data between J=19 and 27) vary with modulation factors increasing from ~1.8 to ~3.2 Impact on mass-loss rate estimates based on 12 CO should therefore be limited if highly excited transitions are not included in the modelling

13 Some highlights CO variability (2) SPIRE 12 CO lines 12 CO J=5-4 HCN v=0 blend HIFI 12 CO lines 12 CO J=10-9 PACS 12 CO lines 12 CO J=16-15 HCN v 1 =1 blend?

14 Conclusions IRC shows strong variation (10-15% up to >1000%) in molecular emission of lines such as CCH, HCN, HNC, CS, SiO, SiS or H 2 O. No variation observed in e.g. SiC 2 12 CO/ 13 CO is non-variable (within calibration uncertainty) at lowto-intermediate transitions, but strongly modulated (factor up to 3) at high energy levels Radiative transfer in molecular lines affected by IR-pumping (typically any high-j line) needs to account for time dependency. However assessment of phase per molecule not trivial Coming up soon: Study of variation time lag between transitions and between blue- and red-shifted portions of the line profiles (HIFI) Constrast Herschel results with monitoring currently on-going at IRAM 30-m in similar species How unique is IRC in that respect: can we detect such variation in other sources? Stay tuned!

15 Summary Species Variability Amplitude factor 12 CO Only for high-j CO Only for high-j 5 HCN v=0 Any transition HCN v 1,v 2,v 3 Any transition H 13 CN v=0 Any transition CS Amy transition SiO Any transition SiS Most but not all! CCH Any transition 4-14 H 2 O Any transition HNC Any transition SiC 2 NO N/A