Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view
LL e tt e r t o t h e E d i t o r Astronomy & Astrophysics manuscript no. version7 c (cid:13)
ESO 2018November 1, 2018 L etter to the E ditor Warm water deuterium fractionation in IRAS 16293-2422
The high-resolution ALMA and SMA view
M. V. Persson , J. K. Jørgensen , and E. F. van Dishoeck Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7,DK-1350, Copenhagen K, Denmarke-mail: [email protected] Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands Max-Planck Institute für extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, GermanyReceived October 29, 2012; accepted November 28, 2012
ABSTRACT
Context.
Measuring the water deuterium fractionation in the inner warm regions of low-mass protostars has so far been hampered bypoor angular resolution obtainable with single-dish ground- and space-based telescopes. Observations of water isotopologues using(sub)millimeter wavelength interferometers have the potential to shed light on this matter.
Aims.
To measure the water deuterium fractionation in the warm gas of the deeply-embedded protostellar binary IRAS 16293-2422.
Methods.
Observations toward IRAS 16293-2422 of the 5 , − , transition of H O at 692.07914 GHz from Atacama LargeMillimeter / submillimeter Array (ALMA) as well as the 3 , − , of H O at 203.40752 GHz and the 3 , − , transition of HDO at225.89672 GHz from the Submillimeter Array (SMA) are presented. Results.
The 692 GHz H
O line is seen toward both components of the binary protostar. Toward one of the components, “source B”,the line is seen in absorption toward the continuum, slightly red-shifted from the systemic velocity, whereas emission is seen o ff -sourceat the systemic velocity. Toward the other component, “source A”, the two HDO and H O lines are detected as well with the SMA.From the H
O transitions the excitation temperature is estimated at 124 ±
12 K. The calculated HDO / H O ratio is (9 . ± . × − – significantly lower than previous estimates in the warm gas close to the source. It is also lower by a factor of ∼ Conclusions.
Our observations reveal the physical and chemical structure of water vapor close to the protostars on solar-systemscales. The red-shifted absorption detected toward source B is indicative of infall. The excitation temperature is consistent with thepicture of water ice evaporation close to the protostar. The low HDO / H O ratio deduced here suggests that the di ff erences betweenthe inner regions of the protostars and the Earth’s oceans and comets are smaller than previously thought. Key words. astrochemistry – stars: formation – protoplanetary disks – ISM: abundances – ISM: general
1. Introduction
Water plays an essential role for life as we know it, but its ori-gin on Earth is still unclear: was water accreted during the earlystages of Earth’s formation, or brought by smaller solar systembodies such as comets at later times? To deduce the origin ofEarth’s water and the amount of chemical processing it has ex-perienced, one option is to measure the water deuterium frac-tionation (HDO / H O) during di ff erent stages in the evolution ofprotostars and compare it to what we measure in Earth’s oceansand comets.Generally the HDO / H O ratio in Earth’s oceans of 3 × − (e.g., Lécuyer et al. 1998) and Oort cloud comets of 8.2 × − (Villanueva et al. 2009) are found to be enhanced above the D / H ratio in the protosolar nebula ∼ . × − (Linsky 2003; Geiss &Gloeckler 1998) due to deuterium fractionation processes. Thefactor of 2 higher abundance ratio in the Oort cloud comets thanin Earth’s oceans has previously been taken as an indication thatonly a small fraction of Earth’s water was delivered by comets.Recently however, a HDO / H O ratio of 3 . × − was measured A HDO / H O ratio of ” x “ corresponds to a D / H ratio of “ x / for the Jupiter class comet Hartley 2 with the Herschel
SpaceObservatory (Hartogh et al. 2011) and 4 . × − for the Oortcloud comet Garradd (Bockelée-Morvan et al. 2012), indicatingvalues closer to those of Earth’s water.Attempts at measuring the water deuterium fractionation inprotostars have resulted in di ff erent conclusions. Parise et al.(2003) used ground-based infrared observations of the stretch-ing bands of OH and OD in water ice in the outer parts of en-velopes and found upper limits ranging from 0.5% to 2% for theHDO / H O ratios in four embedded low-mass protostars. In thegas-phase it is possible to detect lines of HDO, but such studiesdi ff er on the interpretation with HDO / H O ratios in protostarsranging from cometary values (Stark et al. 2004), to a few %(Parise et al. 2005; Liu et al. 2011). Even more recently, Coutenset al. (2012) deduced a HDO / H O ratio in IRAS 16293-2422 of3 . × − in the inner parts and 0 . × − in the outer envelopeby modeling a large range of lines observed with Herschel .One problem with previous measurements of HDO / H O isthe relatively large beam size of single-dish ground- and space-based telescopes. Spherically symmetric power-law models ofprotostellar envelopes have usually been employed to interpret
Article number, page 1 of 6 a r X i v : . [ a s t r o - ph . S R ] N ov e tt e r t o t h e E d i t o r the observations. While such models are appropriate to interpretcontinuum and line emission on larger scales ( >
300 AU), theyare not suited to unambiguously analyze the observed compactcomponents since there are clear indications that they are not anaccurate representation of the conditions on small scales (e.g.,Jørgensen et al. 2005; Chiang et al. 2012). Estimates of abun-dance ratios on these smaller scales are thus subject to signifi-cant uncertainties due to extrapolations of the underlying physi-cal structures.High angular resolution millimeter wavelength aperture syn-thesis observations o ff er a possibility to circumvent this issue.Recently Jørgensen & van Dishoeck (2010) detected the waterisotopologue H O toward the deeply embedded protostar NGC-1333 IRAS4B on scales of <
50 AU using the IRAM Plateau deBure Interferometer (PdBI), which combined with an upper limiton the HDO column density from the SMA resulted in a 3 σ up-per limit to the HDO / H O abundance ratio of 6 × − . To follow-up these results we initiated an extended survey of the H O andHDO emission on arcsecond scales using the IRAM PdBI andSMA (Persson et al. 2012).IRAS 16293-2422 is a Class 0 protostellar binary (sep ∼ (cid:48)(cid:48) ,600 AU) located 120 pc away in the LDN 1689N cloud in the ρ Ophiucus star-forming region (Knude & Høg 1998; Loinardet al. 2008). With a rich spectrum at (sub)millimeter wave-lengths (Blake et al. 1994; van Dishoeck et al. 1995; Cazauxet al. 2003; Chandler et al. 2005; Caux et al. 2011; Jørgensenet al. 2011) it has been one of the prime targets for studiesof astrochemistry during the star-formation process, revealingthe presence of a range of complex organic species (Bottinelliet al. 2004; Kuan et al. 2004; Bisschop et al. 2008) and prebioticmolecules (Jørgensen et al. 2012) on (sub)arcsecond scales.In this letter we present, for the first time, high-resolution ground-based observations of several isotopologues of water to-ward IRAS 16293-2422 with both the ALMA ( ∼ (cid:48)(cid:48) .
2; 24 AU)and the SMA ( ∼ (cid:48)(cid:48) .
3; 276 AU). We derive direct, model-independent estimates of the water excitation temperature anduse it to calculate the column density and the HDO / H O ratio inthe warm inner envelope.
2. Observations
Observations of the deeply-embedded low-mass protostellar bi-nary IRAS 16293-2422 were carried out at 690 GHz withALMA and 230 GHz with the SMA, targeting the 5 , − , (692.07914 GHz) and 3 , − , (203.40752 GHz) transitions ofH O and the 3 , − , (225.89672 GHz) transition of HDO (seeTable A.1 in the appendix / electronic version). In the tables, the203 GHz observations are indicated with “1”, 225 GHz observa-tions with “2” and 692 GHz with “3” (see Table 1 and Table A.1)The ALMA observations of IRAS 16293-2422 were con-ducted as part of the ALMA Science Verification (SV) pro-gram: IRAS 16293-2422 was observed with 13 antennas onApril 16 and 17, 2012 in a seven pointing mosaic centered at α = h m . s δ = − ◦ (cid:48) (cid:48)(cid:48) . λ ). Oneof the basebands was centered at 691.299 GHz with a band-width of 1.875 GHz and a spectral resolution of 0.923 MHz(0.4 km s − ), a setup that covers the H O 5 , − , line at692.07914 GHz. Calibration observations include the quasars1924-292 and 3c279 for the bandpass, the asteroid Juno forthe amplitude, and the quasars 1625-254 and nrao530 for thephase. The science verification data are available as calibrated uv -data sets, which were used in our analysis. The calibrated data were imaged using the CASA software package (McMullinet al. 2007).The lower-lying excited H O 3 , − , transition at203.4075 GHz was observed with the SMA on May 1, 2011 inthe compact configuration with seven antennas. This configura-tion resulted in projected baselines between 9 and 69 meters (6to 47 k λ ). The passband was calibrated by observations of thequasar 3c279 while the absolute flux and complex gains werecalibrated by observing Titan and the quasars 3c279, 1517-243and 1626-298. The raw data calibration followed the standardrecipes using the MIR package (Qi 2008) and then MIRIAD(Sault et al. 1995) was used to subtract the continuum from thedata to create continuum-free line maps.Finally, we utilized SMA observations of the HDO 3 , − , line at 225.89672 GHz from the SMA (Jørgensen et al.2011): those observations have a spectral resolution of 0.41 MHz(0.54 km s − ) and cover the projected baselines between 8 . − . . −
90 k λ ). For further information about those obser-vations we refer to Jørgensen et al. (2011). The resulting beamsize, field of view, velocity resolution and RMS of the variousline data are summarized in Table A.1 in the appendix / electronicversion.
3. Results
Figure 1 shows the spectra around the water lines toward thecontinuum peaks with Gaussian fits. Source A is in itself a bi-nary (Chandler et al. 2005) and we here refer to the componentsas A1 (the Northeast component) and A2 (the Southwest com-ponent) and the lower resolution single component as A. BothH
O lines are clearly detected toward source A in emission,while only the 692 GHz H
O line is seen toward source B inabsorption. The H
O emission is clearly associated with peakA1 in the 692 GHz data. The H
O 692 GHz absorption line to-ward the continuum peak of source B is narrow and marginallyred-shifted; the absorption is not due to the broad outflow seenin some lines (Jørgensen et al. 2011).This weak absorption feature is not seen in the SMA databut the high sensitivity of ALMA combined with the strongercontinuum at high frequencies is enough to detect it. The lineswere identified using the JPL and CDMS catalogs through theSplatalogue compilation (Pickett et al. 1998; Müller et al. 2001).The Splatalogue compilation shows no other possible linecandidates at least ± − from the line peak positions of theH O line at 203 GHz and the HDO line at 225 GHz. Lines thatfall within ± − − exist, but are of too low intrinsic linestrengths or lack additional components that should have beendetected at other velocity o ff sets e.g., (CH ) CO, CH CH CNor C H. At 203.403 and 203.410 GHz (i.e., about 10 and -3 km s − ) lies CH OCH , − , EA / EE, which is accountedfor when fitting the 203 GHz line. These lines are narrow enoughto avoid any significant interference with H
O (see top most plotin Fig. 1).For the H
O ALMA data at 692 GHz the line identificationin source A is complicated by the fact that the source is resolvedand shows a systematic velocity pattern in other lines (Pinedaet al. 2012). The line toward the continuum position of sourceA is red-shifted by about 2 km s − from the systemic v LS R of3.2 km s − , similar to lines from other species (see Fig. 1, panelfour). The third spectrum from the top in Fig. 1 shows the spec-trum after smoothing with a Gaussian kernel of 1 (cid:48)(cid:48) size. TheGaussian fit to these data peaks closer to v LS R = − , in-dicative indeed of resolved velocity structure. Outflow emission Article number, page 2 of 6. V. Persson et al.: Warm water deuterium fractionation in IRAS 16293-2422 L e tt e r t o t h e E d i t o r (c.f., Loinard et al. 2012), as also seen in H O masers on smallscales (Alves et al. 2012), could explain this velocity shift, but itis not possible to deduce its importance from the current data.However, another possibility is that line blending causes theshift. Toward source B, the o ff position spectrum shows twoemission lines, the second line at ∼ − is unidentified (bot-tom spectrum Fig.1). The H O line at 692 GHz toward sourceA may therefore also be a ff ected by line blending. However, theunambiguous detection of the 692 GHz line toward source B andthe SMA observations of water toward source A suggest that atleast half of the 692 GHz feature toward source A is indeed dueto the H O line.Table 1 lists parameters from Gaussian fits to the image and uv -plane of the line emission. The integrated intensity maps forthe detected water lines are shown in Fig. 2. − −
10 0 10 20 300 . . . O 203 GHz A . . . HDO 225 GHz A − . . . . H O 692 GHz A11 Gaussian kernel − . . . H O 692 GHz A1 − . . . H O 692 GHz B − −
10 0 10 20 30v sys [km s − ] − . . . O 692 GHz B Off0 . I n t e n s i t y [ J y b e a m − ] Fig. 1.
Spectra of the targeted water lines toward both IRAS 16293-2422 A and B. The 203 GHz H
O line spectrum (top) was binned totwice the resolution for clarity. The 1D Gaussian fits (red: H
O; green:CH OCH ; magenta: Unidentified) and the RMS (blue) are plotted.Shaded areas show the interval over which the integrated intensitieshave been calculated (yellow and blue). The dotted green vertical linesshows v LSR = . . − for source A and B respectively. Thethird spectrum from the top (blue fill) is from data smoothed with a 1 (cid:48)(cid:48) Gaussian kernel.
4. Discussion
The di ff erent water lines toward source A show similar charac-teristics. The peak positions and line widths of the Gaussianfits to the 203 GHz H O and 225 GHz HDO lines are simi-lar and agree with previous studies of other molecules toward − − −
505 H
O 203 GHzAB200 AU − − −
505 HDO 225 GHz200 AU − . − . − . − . − . − . O 692 GHz AA1A220 AU − . − . . . . O 692 GHz B20 AU
Fig. 2.
Integrated intensity maps for all observed water lines calculatedfrom channels ± − − . Note the di ff erent spatialscales of the top vs the bottom panels. The beam is shown in the lowerright corner and the gray crosses show the position of continuum peaksfrom elliptical Gaussian fits. Units on the axes are o ff set in arcsecondsfrom the phase center of the 203 GHz observations. Contours are insteps of 2 σ starting at 3 σ , dashed contours represent negative values. Table 1.
Parameters from fits to the integrated maps and spectra of thedi ff erent water lines for source A and B. The given errors are the statis-tical uncertainties and the 20% calibration uncertainty in intensity is notincluded. The spectral line fits were done toward the continuum peakand integrated intensities deduced from circular Gaussian fits in the uv -plane (203 and 225 GHz) or an elliptical Gaussian fit in image plane(692 GHz). Line Size Intensity Line widthId [ (cid:48)(cid:48) ] [Jy km s − ] [km s − ]1 (A) 1 . ± . . ± . . ± .
52 (A) 1 . ± . . ± . . ± .
73 (A1) 0 . × . ◦ ) 3 . ± . . ± .
73 (B) Point fit − . ± .
07 0 . ± . Notes.
The column “Line Id” gives the source indicated in parentheses,and the number corresponds to one of the lines (see text and Table A.1).The line width is the FWHM from a Gaussian fit to the spectral line.The elliptical Gaussian parameters (for line 3 (A1)) are given as minorand major axis ( ± (cid:48)(cid:48) .
1) and position angle (PA, ± ◦ ). IRAS 16293-2422 e.g., Bisschop et al. (2008); Jørgensen et al.(2011); Jørgensen et al. (2012); Pineda et al. (2012) (see Ta-ble 1). The mapped emission is compact and traces the warmwater on scales R <
200 AU.Toward source B, the 692 GHz H
O line is detected in ab-sorption. The spectral line is slightly red-shifted compared with v LS R = − of source B, consistent with the picture of on-going infall in this source (Jørgensen et al. 2012; Pineda et al.2012). Integrating over a larger velocity interval reveals, in addi- Article number, page 3 of 6 e tt e r t o t h e E d i t o r tion to the absorption, a blue-shifted emission peak, o ff set ∼ (cid:48)(cid:48) . To calculate the excitation temperature and column densities to-ward source A we assume LTE conditions and that the emis-sion at 203 and 225 GHz has the same extent as in the 692 GHzobservations (see Table 1). Scaling the 203 GHz H
O intensi-ties to the 692 GHz source size gives an excitation temperatureof T ex = ±
12 K for H
O. If the 692 GHz line is indeedblended, it would imply that the intensity estimate, and also theexcitation temperature, are upper limits. Halving the intensityof the 692 GHz line causes a drop to T ex = ± . × cm − , cor-rected for beam dilution when assuming that the extent of theemission is that of the 692 GHz H O observations and T ex =
124 K. The gas-phase H O column density is 5 . × cm − assuming the same as for HDO and that the isotopic abundanceratio of O / O is 560 in the local interstellar medium (Wilson& Rood 1994). Assuming an uncertainty of about 20% for thecolumn densities, originating in the flux calibration, the best es-timate of the HDO / H O ratio is (9 . ± . × − .Given the uncertainty in the determination of the excitationtemperature, testing the e ff ect of di ff erent temperatures is im-portant. If the excitation temperature is as low as 80 K theHDO / H O ratio becomes 7 . ± . × − . Increasing the ex-citation temperature to 300 K (Jørgensen et al. 2012) increasesthe ratio to 1 . ± . × − . The conclusions in this paper donot change over this wide interval in T ex . In addition, since the203 GHz H O and 225 GHz HDO observations have compa-rable beam and sources size, and arise from levels with similarenergies, the deduced HDO / H O ratio is robust. However, if theHDO emission were more extended, its column density and theHDO / H O ratio would be lower. In this scenario our inferredratio is an upper limit.In contrast with previous estimates based on models ofsingle-dish observations (Parise et al. 2005; Coutens et al. 2012),the deduced HDO / H O ratio in the warm gas of IRAS 16293-2422 is only slightly higher than found in Earth’s oceans and byrecent
Herschel observations of comets. Given the possible sys-tematic errors due to assumptions of the extent, they could beeven closer. Within the statistical uncertainties our observed ra-tio for this protostar agrees with the earlier ratios for Oort cloudcomets. Comparing these di ff erent ratios for water directly as-sumes that the reservoirs, i.e., comets, Earth (planet) and innerprotostellar region are linked and can be related.The di ff erence with the earlier estimates comes from the factthat those data are sensitive to much larger scales than the high-resolution interferometric observations presented here, which di-rectly image the water emission on 25 −
280 AU scales. Our in-terferometric observations provide a strong, model independentconstraint on the deuteration of water in the innermost regionsof protostars. That the low HDO / H O ratio in the warm gas isnot much di ff erent from the cometary values is an indication thatsignificant processing of the water between these early stagesand the emerging solar system is not required. Further high-resolution interferometric measurements to-ward larger samples of protostars will reveal whether the warmHDO / H O ratio is similar in di ff erent protostars. In particular,future high angular resolution observations with ALMA will beable to resolve possible variations in the HDO / H O ratio withdistance from the central protostar and thereby show whether theslightly di ff erent ratios measured in di ff erent types of comets po-tentially could be related to their spatial origin in the protostellarenvelope (Robert et al. 2000). Acknowledgements.
We thank the referee, Al Wootten for insightful comments.The research at Centre for Star and Planet Formation is supported by the DanishNational Research Foundation and the University of Copenhagen’s programmeof excellence. This research was furthermore supported by a Junior GroupLeader Fellowship from the Lundbeck Foundation to JKJ. EvD acknowledgesthe Netherlands Organization for Scientific Research (NWO) grant 614.001.008and EU FP7 grant 291141 CHEMPLAN. This paper makes use of the follow-ing ALMA data: ADS / JAO.ALMA / NRAO and NAOJ. The Submillimeter Array is a joint projectbetween the Smithsonian Astrophysical Observatory and the Academia SinicaInstitute of Astronomy and Astrophysics and is funded by the Smithsonian Insti-tution and the Academia Sinica.
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Appendix A: Tables &A–version7,
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Table A.1.
Relevant parameters from the molecular line catalogs for the observed lines.
Line Species Frequency Transition Line strength E u Beam Resolution RMSId a [GHz] [Debye ] [K] Size (PA) [km s − ] [ b ]1 H O 203.40752 3 , − , . (cid:48)(cid:48) × . (cid:48)(cid:48) (-25.3 ◦ ) 0.30 2082 HDO 225.89672 3 , − , . (cid:48)(cid:48) × . (cid:48)(cid:48) (15.0 ◦ ) 0.54 1903 H O 692.07914 5 , − , (cid:48)(cid:48) . × (cid:48)(cid:48) .
29 (109.8 ◦ ) 0.40 82 a Identifies the lines in Table 1. b mJy beam − channel −−