A comparison between the past four decades of astrophysical space missions and those expected to be launched over the next decade shows a marked decrease in numbers. However, missions such as Gaia and the JWST are expected to have a major impact on planetary nebula research. The capabilities of these and other anticipated space missions are discussed

Barlow, MJ

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Planetary Nebulae: an Eye to the FutureProceedings IAU Symposium No. 283, 2012A. Manchado & L. Stanghellini, eds.c© 2012 International Astronomical UnionDOI: 00.0000/X000000000000000XThe Impact of Future Space Observatorieson Planetary Nebula ResearchM. J. Barlow11Dept. of Physics & Astronomy, University College London, Gower Street, London WC1E 6BTAbstract. A comparison between the past four decades of astrophysical space missions and thoseexpected to be launched over the next decade shows a marked decrease in numbers. However,missions such as Gaia and the JWST are expected to have a major impact on planetary nebularesearch. The capabilities of these and other anticipated space missions are discussed.Keywords. planetary nebulae: space observatories1. IntroductionLike many other astrophysical fields, from the 1970’s onwards planetary nebula researchhas benefited greatly from the availability of powerful space observatories operating atwavelengths from the X-ray to the infrared. Table 1 summarises, decade by decade, theprincipal past, current and planned general astrophysics space missions.What are the prospects for the space observatories listed in Table 1 that are currentlystill fully operational? ESA’s Herschel Space Observatory was launched in 2009 and cur-rent projections point to complete helium boil-off, and therefore the end of the mission,occurring in late-2012 or early-2013. For the Hubble Space Telescope the main componentsat risk are believed to be its complement of detectors, with current estimates suggestinga 50% probability by 2015/6 of catastrophic failure due to cumulative radiation damage.At X-ray wavelengths, prospects for the current two observatories operating in spaceare mixed. NASA’s Chandra observatory, launched in 1999, is expected to be able to stayoperational until 2015/6, while the expectations for ESA’s XMM-Newton observatory,also launched in 1999, are better - a recent review has concluded that it could remainoperational till 2025.While six to nine new observatories have been launched in each of the four decadessince 1971, the prospects for the current decade appear bleaker at present, with only fournew general astrophysics missions, GEMS, Astrosat, Gaia and the James Webb SpaceTelescope (JWST), expected to have been launched by 2020.Currently planned for launch in 2014, GEMS, the Gravity and Extreme MagnetismSmall Explorer, will measure X-ray polarization in order to study spinning black holesand probe magnetic field around neutron stars. The other three missions, Astrosat, Gaiaand the JWST, are discussed in more detail below.2. The Astrosat MissionAstrosat is an Indian Space Research Organisation mission for X-ray and UV/opticalastronomy, scheduled for launch in 2012 and expected to have a lifetime in excess of 5years. In addition to having four different X-ray instruments (see Table 2), it will also beequipped with two 40-cm telescopes for UV and optical imaging, each with an angularresolution of 1.8 arcsec and capable of measuring 20th magnitude objects in 1800s. There12 M. J. BarlowTable 1. Past and Future Space Missions for General Astrophysics1Decade X-ray UV or Optical Infrared TOTAL1971-1980 4 4 1 9Ariel-5, SAS-3, TD-1, Copernicus, AFGLHEAO-1, Einstein ANS, IUE1981-1990 3 2 1 6Exosat, Ginga, ROSAT Astron, HST IRAS1991-2000 4 3 2 9ASCA, RXTE, Chandra EUVE, FUSE ISO, MSXXMM-Newton Hipparcos2001-2010 1 1 4 6Suzaku GALEX Spitzer, AKARI,Herschel, WISE2011-2020 2 2 (1) (5)GEMS, Astrosat Astrosat, GAIA JWST∼2014, 2012 2012, 2013 2018?TOTAL 14 12 (9) 35Post-2020 ATHENA? WSO? SPICA?1 Missions with particular impact on planetary nebula research are boldfaced.Table 2. Astrosat X-ray and UV/Optical Science InstrumentsInstrument Energy Range Description Ang. Res. SensitivitySXT 0.3-8 keV FOV=42′ 3-4 arcmin 0.01 mCrab in 104sLAXPC 2-80 keV 3 Prop. Counters 5 arcmin 0.1 mCrab in 103sCZT 10-150 keV Hard X-ray Imager 8 arcmin 0.5 mCrab in 103sSSM 2-10 keV Scanning Sky Monitor 5-10 arcmin 50 mCrab in 300sUVIT 1300-5500 Å Twin 40-cm Telescopes 1.8 arcsec 21st mag in 1800swill be one broad-band far-UV filter (1300-1500 Å), four broad-band and two narrow-band near-UV filters, plus four broad-band filters in the 3000-5500-Å region. Furtherdetails are available at www.iucaa.ernet.in/∼astrosat/. Given the very low number ofspace missions currently planned for the X-ray and UV/optical domains, Astrosat is awelcome additional resource, with potential interest for studies of PN central stars.3. GaiaGaia is an ambitious ESA mission which aims to measure parallaxes and proper motionsfor one billion stars out to distances of 50 kpc. During its L2 five-year mission, beginningin 2013, it will survey the whole sky in order to achieve astrometric completeness downto V=20. Its gigapixel imaging system consists of 106 CCD sensors each with 1966×4500Future Space Observatories 3pixels. Final positional accuracies will be 7, 20 and 300 µarcsec at V = 10, 15 and 20,respectively. Distances accurate to 1% are expected to be obtained for 107 stars out to2.5 kpc, as well as distances accurate to 10% for 108 stars out to 25 kpc. Many PN centralstars will therefore have accurate distances available by the end of the Gaia mission. Themost favourable cases will be for central stars in low surface brightness nebulae, i.e. oldPNe, whereas central stars in higher surface brightness PNe will be more problematic(see below).Gaia has twin telescopes, each with a with rectangular 1.49×0.54m primary, with afixed wide angle between their lines of sight (LOS). The mirrors in each telescope systemare made of sintered silicon carbide, with enhanced silver reflective coatings to optimisetheir reflectivities in the 3200-10000 Å region. The spacecraft will rotate around an axisperpendicular to the LOS’s of the two telescopes, with a rigid frame of reference beingobtained by measuring the relative positions of objects from both observing directions.Each object will be measured about 70 times during the 5-year mission.Gaia will have three instruments: (1) an astrometric instrument dedicated to measuringthe angular positions of stars with V magnitudes in the range 6-20; (2) a photometricinstrument covering the same magnitude range, which will acquire spectrophotometry atultralow spectral resolution and will subsequently convolve the spectra with a range offilter pass-band profiles to generate photometry in 4 broad bands and 11 medium bands,including an Hα band; (c) a high-resolution spectrometer covering the 8470-8740 Å regionat R=11,500, which will use the Ca ii infrared triplet lines and the H i Paschen-14 line tomeasure stellar radial velocities to an accuracy of better than 15 km s−1 down to V=17.Observations from the photometric instrument should be of particular interest to plan-etary nebula research. As Gaia rotates across the sky, the spectrophotometer splits theincoming light and sends it through prisms to blue- and red-sensitive CCDs, which willrespectively cover the 3300-6600 Å and 6500-10500 Å wavelength ranges. The result-ing spectra will have a dispersion which is a function of wavelength, ranging from 40-320 Å pixel−1 in the blue and 70-150 Å pixel−1 in the red. These spectra have thepotential to discover a significant number of young compact PNe and post-AGB objectswith emission lines. However, current plans are that data on objects found to have an-gular diameters larger than 0.7 arcsec will be thrown away, i.e. not transmitted to theground, which will clearly impact on prospects for measurements of PNe with mediumto high nebular surface brightnesses.4. The James Webb Space TelescopeAs is well known, the JWST is aimed to be NASA’s flagship successor to the HubbleSpace Telescope, with a 15% funding contribution from ESA. It is currently beset bypolitical problems in the US, with a House sub-committee attempting in July 2011 toaxe the project, after ∼ $4bn had already been spent. However, the equivalent Senatesub-committee restored funding for the JWST in their version of the spending bill andit appears that it may survive in an eventual reconciled bill. However, budgetary issueshad already forced a delay in its expected launch date, from 2014 to 2018.The JWST will have a deployable segmented-mirror primary, equivalent in collectingarea to a single mirror with a 6.55m diameter, which will use passive cooling to achievean operating temperature of ∼45 K at its L2 station, versus ∼80 K for the 3.5-m mirrorof the Herschel Space Observatory. The JWST will have four instruments, a 0.6-5 µmnear-IR wide field camera (NIRCam), which will have a coronagraphic mask available;a 1-5 µm near-IR multi-object spectrometer (NIRSpec); a 5-18 µm mid-IR camera andspectrometer (MIRI), also with a coronagraphic mask available; and a Tunable Filter4 M. J. BarlowTable 3. JWST Instrument PropertiesInstrument λ-range Features Detectors Platescale FoV(µm) (arcsec)NIRCam (SW) 0.6-2.3 R∼4, 10, 100 filters Two 2×2 mosaics 32 2.2×4.4′+coronagraph of 2048×2048NIRCam (LW) 2.4-5.0 R∼4, 10, 100 filters Two 2×2 mosaics 65 2.2×4.4′+coronagraph of 2048×2048NIRSpec R=100 0.6-5.0 Slits/microshutters Two 2048×2048 100 3.4×3.1′NIRSpec R=1000 1.0-5.0 Slits/microshutters Two 2048×2048 100 3.4×3.1′NIRSpec R=3000 1.0-5.0 Integral Field Unit Two 2048×2048 100 3.0×3.0′MIRI (Imaging) 5-28 Filters, coronagraph 1024×1024 110 1.4×1.9′MIRI (Prism) 5-10 R=100 1024×1024 110 1.4×1.9′MIRI (Echelle) 5-28 IFU R=3000 Two 1024×1024 220-470 3.6-7.5”FGS-TFI (SW) 1.5-2.5 R∼100 etalon 2048×2048 68 2.3×2.3′FGS-TFI (LW) 3.2-5.0 R∼100 etalon 2048×2048 68 2.3×2.3′Imager (TFI), associated with the optical bench of the Fine Guidance Sensor (FGS) andcovering the 1.5-2.5 and 3.2-5.0 µm regions with a resolving power of 75-120. Table 3summarises the main properties of the four science instruments on the JWST, whileFig. 1 compares the expected point source sensitivities of the NIRCam and MIRI imagerswith those of several other spaceborne, airborne and ground-based telescopes. At mid-IRwavelengths Spitzer went nearly a factor of 100 deeper than ground-based 8-m telescopes,while JWST will go a factor of 50 deeper than Spitzer. Since young PNe and post-AGBobjects usually show strong dust continuum, dust feature and line emission in the 10-20-µm region, they should be easily detectable out to very large distances with the JWST.JWST’s huge sensitivity gains have one important consequence, however, namely thatits saturation limits will be at unprecedently faint levels, preventing observations of manysources in the Milky Way and Magellanic Clouds. For example, even MIRI’s R=3000spectroscopic mode will saturate on sources with 10-µm fluxes of only ∼0.5 Jy. TheLMC post-AGB object SMP 11, which shows a rich mid-IR absorption spectrum fromorganic molecules such as benzene (Bernard-Salas et al. 2006), has a 10-µm flux only justbelow this level. Photometric studies of AGB stars, post-AGB objects and PNe usingNIRCam and MIRI will often only be feasible for targets lying at greater distances thanthe Magellanic Clouds.LMC SMP 11 would be detectable at 10σ in 104 sec out to a distance of 18 Mpcat 18 µm; out to a distance of 38 Mpc at 10 µm; and out to a distance of 47 Mpc at7.5 µm. Objects discovered via imaging photometry could be followed up from 5-10 µmwith MIRI’s long-slit R = 100 spectrometer. SMP LMC11 would yield 10σ per spectralresolution element in 104 sec at a distance of 14.5 Mpc. These numbers indicate that inthe above modes MIRI should be comfortably capable of studying similar objects out tothe Virgo Cluster (D ∼ 14 Mpc) and beyond.Planetary nebulae have become important probes of extragalactic stellar systems, usingthe bright [O iii] 5007 Å line, in which up to 10% of the nebular luminosity can beemitted. Arnaboldi et al. (1996) measured velocities for 19 PNe in the outer regions ofthe giant elliptical galaxy NGC 4406, in the southern Virgo extension region. AlthoughFuture Space Observatories 5Figure 1. 104s 10-σ point source detection limits for JWST’s NIRCam and MIRI instruments,compared to those for HST, Spitzer, SOFIA and 8-m Gemini instruments, illustrating the verylarge sensitivity gains anticipated.Figure 2. Spitzer-IRS hi-res spectra of (left) SMP LMC 8, showing a strong SiC 11.2-µmemission feature and (right) SMP LMC 36, showing very strong PAH emission bands. Bothplanetary nebulae also exhibit narrow fine structure ionic emission lines in their spectra. TheIRS lo-res spectra of these PNe are shown by Bernard-Salas et al. (2009)this galaxy has a peculiar radial velocity of −227 km s−1, three of the PNe had velocitiesclose to 1400 km s−1, the mean radial velocity of the Virgo cluster. It was concludedthat they were intracluster PNe. Theuns & Warren (1997) discovered ten PN candidatesin the Fornax cluster, in fields well away from any Fornax galaxy - consistent with tidalstripping of cluster galaxies. They estimated that intracluster stars could account for upto 40% of all the stars in the Fornax cluster. Méndez et al. (1997) surveyed a 50 arcmin2area near the centre of the Virgo cluster, detecting 11 PN candidates. From this, theyestimated a total stellar mass of about 4×109 M⊙ in their survey area and that such apopulation could account for up to 50% of the total stellar mass in the Virgo cluster.The luminous intergalactic PNe discovered to date at optical wavelengths therefore6 M. J. BarlowFigure 3. Predicted point source photometric detection sensitivities, in µJy (5-σ, 1-hour), forSPICA, compared to those for the JWST, Herschel and ALMA. From Swinyard & Nakagawa(2009).represent the tip of a very large iceberg. Infrared observations with JWST of dust-emitting AGB stars and planetary nebulae located in intergalactic regions of the VirgoCluster appear feasible. Figure 2 shows the Spitzer mid-infrared spectra of two LMCplanetary nebulae. The fluxes measured for the SiC-emitting nebula SMP LMC 8 indicatethat in 104 sec MIRI imaging would yield 10σ detections for a distance of D = 19 Mpcwith the 7.7- and 11.3-µm filters and for a distance of 13 Mpc with the 18-µm filter. Forthe strongly PAH-emitting PN SMP LMC 36 (Fig. 2), in 104 sec MIRI imaging wouldyield a 10σ detection for a distance of 44 Mpc with the 7.7-µm filter and for a distanceof 23 Mpc with the 11.3-µm filter. The carbon star MSX SMC 159 (Sloan et al. 2006),would yield 10σ detections in 104 sec with MIRI out to a distance of D =29 Mpc withthe 5.6-µm filter, to 23 Mpc with the 7.7-µm filter and to 12 Mpc with the 11.3-µm filter.Similar detection limits apply for oxygen-rich AGB stars. As shown by Groenewegen etal. (2007), mid-infrared colour indices can be used to estimate mass loss rates and therebymass inputs from stellar populations hosting AGB stars.The other JWST instruments will also be useful for such studies, e.g. with the Tun-able Filter Imager (R = 100) a carbon star similar to MSX SMC 159 would give 10σper resolution element in 104 sec for distances out to D = 24-29 Mpc, for wavelengthsfrom 3 µm. There are at least 20-30 AGB stars for every PN, so there should be manydetectable AGB stars in Virgo Cluster fields, allowing the total stellar population to bedetermined, as well as their gas and dust mass inputs into the intracluster medium.5. Other Future MissionsSPICA is a Japanese-led infrared mission, with ESA participation, which is currently atan advanced design stage, awaiting Phase B approval. It will cover the 5-210-µm wave-length range with a 3.5-m primary cooled to less than 5 K by mechanical cryocoolers,giving it a similar mid-IR sensitivity to that of the JWST, despite its smaller primary, andFuture Space Observatories 7a far-IR sensitivity a factor of 100 better than Herschel’s, see Figure 3. To be launched toan L2 station by the end of the decade, it has a mission lifetime goal of 5 years, potentiallyextendable to 10 years. It will be equipped with a mid-IR imaging spectrophotometer andcoronagraph, a mid-IR medium and high resolution spectrometer and a far-IR imagingspectrometer. As well as supplying the telescope, ESA would provide the far-IR imag-ing spectrometer, called SAFARI. SPICA’s wide-field mid-IR and far-IR cameras andimaging spectrometers will give it a powerful capability to detect IR continuum- andline-emitting objects in blind surveysThe 2.4-m HST’s STIS and COS spectrometers currently provide the sole ultravioletspectroscopic capability to astronomers. However, UV spectroscopic observations receiveonly a minority of the total time allocated on the the HST and beyond 2015/6 Hubble’scapabilities may start to fail. There is a clear need for a dedicated UV spectroscopic mis-sion. The ‘World Space Observatory’ (WSO) is based on a concept that was pioneered andvigorously pursued by the late Willem Wamsteker and is currently a Russian-led inter-national collaboration, see http://wso.inasan.ru/. Its 1.7-m telescope would be equippedwith R=50,000 spectrometers (Germany) covering the 1020-3100-Å region, a long-slitspectrometer (China) and cameras with a 2.2′-3.4′ field of view for 1200-6000-Å UV andoptical imaging (Spain). Its UV-optimised mirror coatings would give it an ultravioletspectroscopic performance as good as or better than that of the Hubble Space Telescope.Its operational lifetime in geosynchronous orbit would be 5 years, with a 10-year goal.6. Some Conclusions• In common with many other astrophysical research fields, planetary nebula studiesneed multi-wavelength spectroscopic and imaging coverage from space. Planetary nebularesearchers therefore need to make a full contribution to future mission definition studies.• Cancellation of the James Webb Space Telescope could have catastrophic conse-quences for US space science and technology leadership.• The decrease in the number of astrophysics space missions per decade, evident fromTable 1, needs to be reversed. This may imply proposing somewhat less ambitious andcertainly less costly missions in the future.• New missions should aim for operational lifetimes of 10-15 years.• Space agencies appear not to like long-term operational commitments. Astronomersshould consider pushing for other bodies to take over responsibility for post-launch op-erations of future missions, e.g. to take effect circa 3-5 years after launch.ReferencesArnaboldi, M., Freeman, K. C., Mendez, R. H., Capaccioli, M., Ciardullo, R., Ford, H., Gerhard,O.; Hui, X., Jacoby, G. H., Kudritzki, R. P., Quinn, P. J. 1996, ApJ, 472, 145Bernard-Salas, J., Peeters, E., Sloan, G. C., Cami, J., Guiles, S., Houck, J. R. 2006, ApJ, 652,L29Bernard-Salas, J., Peeters, E., Sloan, G. C., Gutenkunst, S., Matsuura, M., Tielens, A. G. G.M., Zijlstra, A. A., Houck, J. R. 2009, ApJ, 652, L29Groenewegen, M. A. T., et al. 2007, MNRAS, 376, 313Méndez, R. H., Guerrero, M. A., Freeman, K. C., Arnaboldi, M., Kudritzki, R. P., Hopp, U.,Capaccioli, M., Ford, H. 1997, ApJ, 491, L23Sloan, G. C., Kraemer, K. E., Matsuura, M., Wood, P. R., Price, S. D., Egan, M. P. 2006, ApJ,645, 1118Swinyard, B., Nakagawa, T. 2009, Exp. Astr., 23, 193Theuns, T., Warren, S. J. 1997, MNRAS, 284, L11

The impact of future space observatories on planetary nebula research

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The impact of future space observatories on planetary nebula research

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