- Email: [email protected]
26Al emission throughout the Galaxy
New Astronomy Reviews 52 (2008) 440–444
Contents lists available at ScienceDirect
New Astronomy Reviews journal homepage: www.elsevier.com/locate/newastrev
26
Al emission throughout the Galaxy
Roland Diehl *, Michael Lang, Karsten Kretschmer, Wei Wang Max Planck Institut für extraterrestrische Physik, D-85748 Garching, Germany
a r t i c l e
i n f o
Article history: Available online 24 June 2008 PACS: 97.60. s 97.10.Cv 26.20.+f 26.30.+k 95.30.Cq 98.38. j
a b s t r a c t Gamma-rays from decay of 26Al have established that nucleosynthesis is currently ongoing in the Galaxy and that nuclear reactions inside massive stars and their supernovae are the prime origins of this isotope, which is rather short-lived compared to time scales of Galactic evolution. The high-resolution spectrometer ‘SPI’ on INTEGRAL is carrying out a survey of the Galactic plane in 26Al gamma-rays, aiming to add spectroscopic details which constrain the kinematics of decaying 26Al nuclei. First results have shown the Galaxy-integrated gamma-ray line to be narrow and compatible with expected ISM velocities at or below 100 km/s. More exposure will allow increasingly more localized and spatially-resolved studies. Line centroid shifts in the inner Galaxy have been found to be consistent with the large-scale Galactic rotation. Here we add first results from spatially-resolved 26Al line spectroscopy along the Galactic plane. Ó 2008 Published by Elsevier B.V.
Keywords: Stars Late stages of evolution Nucleosynthesis Nuclear processes (astrophysics) Interstellar matter Milky way
1. Introduction Gamma-rays from the decay of radioactive by-products of nucleosynthesis ejecta rather directly reflect ongoing nucleosynthesis. For an isotope such as 26Al with decay lifetime much longer than the separation between the cosmic nucleosynthesis events (such as supernovae or the Wolf-Rayet-phase of massive star evolution), cumulative emission will be observed from the abundance of this isotope in the interstellar medium. This results from the balance of decay and the production from all kinds of sources. In these cases of long-lived isotopes such as 26Al and 60Fe, gamma-rays from radioisotopes also should provide insights into the state of the interstellar medium which would be reflected in the measured line position and shape due to the Doppler effect. High-resolution solid state detectors in space, e.g. RHESSI (Lin et al., 2003), and INTEGRAL/SPI (Vedrenne et al., 2003; Roques et al., 2003; Winkler et al., 2003) have added this new quality to the ‘‘astronomy with gamma-ray lines”: Fine spectral resolution allows to measure the Doppler shifts, which are proportional to the photon energy and amount to keV-sized effects for 100 km/s velocities. From the first generation of measurements (HEAO-C, SMM, COMPTEL; see review by Prantzos and Diehl, 1996) we have * Corresponding author. Tel.: +49 89 30000 3850. E-mail address: [email protected] (R. Diehl). 1387-6473/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.newar.2008.06.024
learned that 26Al gamma-ray emission is extended over larger regions of the sky and, more precisely, distributed along the plane of the Galaxy with indications of enhancements in spiral-arm tangent directions and the Cygnus region (Diehl et al., 1995) (see Fig. 1). Since sources with individually-low contributions but high frequency of occurrence, such as novae or low-mass AGB stars, would be expected to result in a smooth spatial distribution of nucleosynthesis products in the Galaxy, this was taken as a main argument to favor massive stars as dominating sources. In a steady-state situation, massive-star contributions from core-collapse supernovae are indistinguishable from those during the WolfRayet phases of such stars (see Fig. 2). Therefore, attempts to disentangle these two nucleosynthesis sites have been based on theory arguments, using models for explosive nucleosynthesis or Wolf-Rayet stars to estimate the expected total yields. The suggested differences in yield dependencies from metallicity for supernovae and Wolf-Rayet stars, if combined with the observationally-indicated Galactic metallicity gradient, may provide an observational diagnostic, especially if combined with other observables from these types of sources (see Knödlseder, 1999). Deviations from a global steady state are expected for individual massive-star regions, as it is well known that massive stars preferentially are created in clusters as coeval groups of stars. In such stellar associations, depending on their age, Wolf-Rayet or core-collapse ejections of 26Al could dominate, for age intervals
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
3–5 and >5 Myrs, respectively (Plüschke et al., 2000, 2002; Cervino et al., 2000). The Cygnus region has been a prominent target of such studies, even though altogether nine OB associations are seen in overlapping projections. From population synthesis modeling it appears that Wolf-Rayet stars dominate 26Al production in this region (Plüschke et al., 2000; Cervino et al., 2000). INTEGRAL currently devotes a key program to this science objective, with altogether 10 Ms of exposure in the Cygnus region of the Galaxy. Other candidate regions with observationally-distinguished massive-star clusters are found in the Vela, Orion, and Sco-Cen regions (cmp. Fig. 1), and are also targets of specific 26Al studies. Here we summarize findings on the large-scale Galactic 26Al emission from the present INTEGRAL database and we report on the status and first results from spatially-resolved analysis.
2.
26
Al throughout the Galaxy
Various instruments have measured the large-scale 26Al emission from the inner regions of the Galaxy since its first detection in 1978 with the HEAO-C experiment (see review by Prantzos and Diehl, 1996). The measured fluxes, as normalized to the ‘‘inner Galaxy”, are roughly consistent and scatter around a value of 4 10 4 ph cm 2 s 1 for the central radian (which is a conventional reference and normalization, roughly covering the brightest regions in the sky, except for Cygnus). Imaging instruments appear to fall somewhat lower in their reported flux values: INTEGRAL/SPI measured a flux value of 3.3 (±0.4) 10 4 ph cm 2 s 1 (Diehl et al., 2006), while COMPTEL’s value was 2.8 (±0.4) 10 4 ph cm 2 s 1 (Plüschke et al., 2001). This may be an indication of either different
441
susceptibilities to instrumental background with a common feature near the 26Al line, or possibly an indication for extended-nearby or isotropic 26Al emission. In any case, depending on the adopted distribution of 26Al in 3-dimensional models of the Galactic sources, this translates into a total Galactic 26Al mass of 2.8 (±0.8) M . Here INTEGRAL/SPI added an argument from high-resolution spectroscopy of the 26Al line towards different directions in the inner Galaxy: The line shifts indicated by separating three different directions appeared consistent with what is expected from plausible models for spatial distribution of sources and rotational pattern in the Galaxy (Kretschmer et al., 2003). This supports the interpretation of the observed flux in terms of large-scale integrated emission along the full line of sight throughout the Galaxy, rather than possible systematic distortions from peculiar bright regions such as Cygnus at unknown but more nearby distances. It reaffirms earlier conclusions about the total Galactic amount of 26Al present in the current-day interstellar medium. Different astrophysical views of this 26Al mass measurement can be taken (see Figs. 3a,3b and discussion in caption). If massive stars are held responsible for this steady-state production, a combination of theoretical yields per mass of the star and a distribution function for numbers of stars per stellar-mass interval determines a steady-state rate of star formation, or, equivalently, a steadystate rate of core-collapse supernova events in the present-day (last few Myrs) Galaxy. From their balloon experiment measurement the GRIS team had reported an intrinsic line width of 5.4 keV (Naya et al., 1996), the celestial broadening of which would have corresponded to interstellar gas velocities of 500 km s 1. This was difficult to
Fig. 1. The Galaxy has been recognized as a diffuse source of gamma-rays at 1808.65 keV energy (Diehl et al., 1995) from the radioactive decay of 26Al (s 1 My), produced and ejected from nucleosynthesis sources throughout the Galaxy. The image presents the results from 9 years of observations with the COMPTEL telescope aboard NASA’s Compton Observatory (1991–2000; Plüschke et al., 2001). The spectrum shown in the upper right results from observations with the SPI Ge Spectrometer on ESA’s INTEGRAL Mission (since 2002) (Wang et al., in preparation; Diehl et al., 2006a). From the irregular appearance of the COMPTEL image and other arguments, it had been concluded that massive stars probably dominate the Galactic production of 26Al (see Prantzos and Diehl, 1996). From the narrow line shape observed with INTEGRAL, it is concluded that 26Al propagates in its source surroundings to obtain normal interstellar-medium velocities, on average (Diehl et al., 2006b).
442
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
Fig. 2. The final evolutionary phase of a massive star, shown in logarithmic time scale with the core collapse event at the rightmost end (adapted from A. Heger). This ‘‘Kippenhahn diagram” reflects the complex structure and energy production & transport within a star, the ordinate being the logarithmic mass coordinate with stellar core at the bottom and envelope at the top. Shaded regions denote nuclear energy production and neutrino energy losses, respectively, while hatched regions indicate where convection is responsible for energy transport. Mass loss sheds the outer envelope in later phases, carrying 26Al products from the early core hydrogen burning phase as it was mixed with the envelope. 26Al production which occurs later in the Si burning shell or during explosive O/Ne burning will be ejected in the terminal supernova explosion.
18
SFR [Moy-1]
15 12 9 6 3
G üs
Sm ith
e
ta l.1 97 Ta 8 lb ot te 19 n 8 et 0 al .1 98 Tu 2 rn er 19 M 8 ez 4 ge r1 98 M cK 7 ee va 19 M n 8 va cK 9 de n ee n de Be 19 n rg 89 Be h R rg & ad h Ta io 19 m Su 90 m pe an H rn n is ov to 19 ric a 91 R Su em pe na rn va C nt ov ap n s a pe de R lla n ec Be ro or rg et d h al & .1 M 9 cC 93 lu re M 19 cK Ti 94 Pa m ee m ge & es l1 W ,D 9 illi 94 ie am hl ,H s 19 ar 97 tm St an ah n le 19 r& 97 Pa lla 20 04 Ku nd t2 00 R 4 ee D d ie 2 00 hl et 5 al .2 00 6
0
Fig. 3a. Key parameters of our Galaxy, as derived from the determinations of the current Galaxy’s content of 26Al isotopes: The gamma-ray line measurement, combined with massive-star model yields and the stellar initial mass function yields a new method to determine the supernova rate from core collapses (above; rightmost data column). As indicated, uncertainties are large in estimating this quantity, since extragalactic as well as solar-vicinity-based estimations carry substantial systematic uncertainties from extrapolation towards the Galaxy. (from Diehl et al., 2006b).
understand (Chen et al., 1997) and is now clearly ruled out from results obtained with RHESSI (Smith, 2003) and SPI (Diehl et al., 2003). More data help to further constrain the line width, improving the 95% probability value from <2.8 keV (Diehl et al., 2003) to
1.3 keV (Wang et al., 2008; see Fig. 4; flattening of the probability distribution below 0.4 keV may indicate a true line broadening of a few tenths of a keV).It may be possible with further years of observation to obtain a width determination, rather than an upper limit.
443
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
350
30
Solar System Interstellar Grains 300
25
250 20 200
ISM γ rays
15
150 10 100
5
50
0 0 .0 -7
0 0 .0 -6
0 .0 -5
0 .0 -4
0 .0 -3
0 .0 -2
0 .0 -1
00 0.
Fig. 3b. Key parameters of our Galaxy, as derived from the determinations of the current Galaxy’s content of 26Al isotopes: The isotopic ratio 26Al/27Al for the current-day Galaxy derives from the gamma-ray measurement and the solar Al abundance attributed to the Galaxy’s ISM mass estimate (below; hatched area). This value is lower than the isotopic ratios found in extra-solar grains embedded in meteorites, which are assumed to condense out of nucleosynthetically-enriched ejecta near the source. The gamma-ray measurement also falls below the value determined from the variety of isochrones of solar-system meteorites, which formed 4.6 Gyrs ago. The metallicity (hence 27 Al abundance) presumably was lower then, and additionally it is believed that the solar system experienced a peculiar initial enrichment in 26Al. (from Diehl et al., 2006b).
Fig. 4. Constraints on celestial broadening of the 26Al line: The probability distribution is asymmetric, and only an upper limit can be determined. Over 4 years of INTEGRAL data, now yield a 95% limit below 1 keV, corresponding to Doppler velocities slightly above 100 km/s (Wang et al., in preparation).
3. Spatially-resolved
26
Al studies
INTEGRAL observations focused on the inner Galaxy during the initial mission years, extending exposure towards larger longitudes thereafter. Several key programs were selected which aim at largescale diffuse-emission or source population studies, and it is one of the mission goals to assemble a legacy database for Galactic-plane survey studies. The variation of 26Al emission along the plane of the Galaxy is a prominent science goal. Continuing the work performed in the inner Galaxy (Diehl et al., 2006a,b) which had obtained the hints for Doppler shifts according to the Galaxy’s large-scale rotation, we now have obtained first line centroid and width results for the bright ridge of 26Al emission (Wang et al., 2008) (cmp. Fig. 1).
Fig. 5 shows spectra obtained for different longitude intervals for different spatial bins. Regions of interest for localized studies have so far concentrated on groups of massive stars, which are well separated from the underlying disk population of the Galaxy; examples are the Cygnus region as well as the Vela and Orion regions. With deep exposure in the central directions of our Galaxy and the fourth quadrant, it now also appears promising to target the well-known nearby stellar groups of the Sco-Cen association. A hint of 26Al emission from these stars had been suggested from the COMPTEL image. This association has been suggested to be a prominent nearby laboratory for star formation triggered by successive generations of stars, due to their impact on nearby molecular clouds (Preibisch and Zinnecker, 1999), with latest star forming episodes being at the origin of the tenuous hot interstellar cavity around the Sun (e.g., Frisch, 1981 and follow-up work). SPI data from this region can barely be separated from the nearby bright inner-Galaxy region. Yet, first spectra clearly suggest 26Al emission, with enhanced brightness of 26Al emission north of the Galactic plane, when directly compared to its southern counterpart. From the localized spectra shown in Fig. 5, the Aquila region near longitudes 30° possibly is peculiar, and deserves further study. Additionally, a dedicated 3 Ms exposure of the Orion region has recently been completed and is being studied. In Orion, earlier stellar subgroups of the OB1 association have blown a cavity which extends towards us, and may be guiding the streaming of nucleosynthesis ejecta from currently-active nucleosynthesis subgroups. This is indicated in imaging studies of 26Al gamma-rays with COMPTEL (see Diehl, 2002). In summary, INTEGRAL’s spectral resolution and deep exposures along the plane of the Galaxy have significantly advanced 26Al studies in the Galaxy: large-scale results from COMPTEL imaging have been confirmed, and complemented by constraints on the average kinematic properties of the ISM surrounding 26Al source regions. More specific insights are at the horizon for localized emission
444
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
Fig. 5. Spatially resolved determinations of the 26Al gamma-ray line parameters (Wang et al., in preparation). Although the statistical quality of these spectra is marginal, some indications for variations of line properties (centroid, width) can be seen. Upon deeper studies, this bears the potential to identify regions with peculiar kinematic properties such as bulk motion or increased line broadening due to enhanced ISM turbulence from, e.g., young wind-blown cavities.
regions, where both the source populations and ISM environments can be usefully constrained from other astronomical observations. Acknowledgements This contribution results from the collaborative work with the members of the INTEGRAL and SPI Teams, and fruitful discussions with many other colleagues; we are grateful in particular to Hubert Halloin and Andrew Strong at MPE, Pierre Jean, Jürgen Knödlseder, and Jean-Pierre Roques at CESR, and Dieter Hartmann at Clemson University. INTEGRAL is an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA. The SPI spectrometer has been completed under the responsibility and leadership of CNES/France, its anticoincidence system is supported by the German government through DLR grant 50.0G.9503.0. We acknowledge the support of INTEGRAL from ASI, CEA, CNES, DLR, ESA, INTA, NASA and OSTC.
References Cervino, M., Knödlseder, J., Schaerer, D., et al., 2000. A&A 363, 970. Chen, W. et al., 1997. ESA-SP 382, 105. Diehl, R., Dupraz, C., Bennett, K., et al., 1995. A&A 298, 445. Diehl, R. et al., 2003. A&A 411, L451. Diehl, R., Halloin, H., Kretschmer, K., et al., 2006a. A&A. Diehl, R., Halloin, H., Kretschmer, K., et al., 2006b. Nature 439, 45. Diehl, R., 2002. New Astron. Rev. 46, 8–10. 547. Frisch, P., 1981. Nature 293, 377. Knödlseder, J., 1999. ApJ 510, 915. Kretschmer, K., Diehl, R., Hartmann, D.H., 2003. A&A 412, 47. Lin, R.P. et al., 2003. Sol. Phys. 210, 3. Naya, J.E. et al., 1996. Nature 384, 44. Plüschke, S., Cervino, M., Diehl, R., et al., 2002. New Astron. Rev. 46, 535. Plüschke, S., Diehl, R., Wessolowski, U., et al., 2000. AIP 510, 44. Plüschke, S., Diehl, R., Schönfelder, V., et al., 2001. ESA-SP 459, 55. Prantzos, N., Diehl, R., 1996. Phys. Rep. 267 (1), 1. Preibisch, T., Zinnecker, H., 1999. A.J. 117, 2381. Roques, J.-P., Schanne, S., von Kienlin, A., et al., 2003. A&A 411, L91–L100. Smith, D., 2003. ApJ 589, L55. Vedrenne, G., Roques, J.-P., Schönfelder, V., et al., 2003. A&A 411, L63–L70. Wang, W. et al., 2008. A&A, manuscript in preparation. Winkler, C., Courvoisier, T.J.-L., DiCocco, G., et al., 2003. A&A 411, L1–L6.
Contents lists available at ScienceDirect
New Astronomy Reviews journal homepage: www.elsevier.com/locate/newastrev
26
Al emission throughout the Galaxy
Roland Diehl *, Michael Lang, Karsten Kretschmer, Wei Wang Max Planck Institut für extraterrestrische Physik, D-85748 Garching, Germany
a r t i c l e
i n f o
Article history: Available online 24 June 2008 PACS: 97.60. s 97.10.Cv 26.20.+f 26.30.+k 95.30.Cq 98.38. j
a b s t r a c t Gamma-rays from decay of 26Al have established that nucleosynthesis is currently ongoing in the Galaxy and that nuclear reactions inside massive stars and their supernovae are the prime origins of this isotope, which is rather short-lived compared to time scales of Galactic evolution. The high-resolution spectrometer ‘SPI’ on INTEGRAL is carrying out a survey of the Galactic plane in 26Al gamma-rays, aiming to add spectroscopic details which constrain the kinematics of decaying 26Al nuclei. First results have shown the Galaxy-integrated gamma-ray line to be narrow and compatible with expected ISM velocities at or below 100 km/s. More exposure will allow increasingly more localized and spatially-resolved studies. Line centroid shifts in the inner Galaxy have been found to be consistent with the large-scale Galactic rotation. Here we add first results from spatially-resolved 26Al line spectroscopy along the Galactic plane. Ó 2008 Published by Elsevier B.V.
Keywords: Stars Late stages of evolution Nucleosynthesis Nuclear processes (astrophysics) Interstellar matter Milky way
1. Introduction Gamma-rays from the decay of radioactive by-products of nucleosynthesis ejecta rather directly reflect ongoing nucleosynthesis. For an isotope such as 26Al with decay lifetime much longer than the separation between the cosmic nucleosynthesis events (such as supernovae or the Wolf-Rayet-phase of massive star evolution), cumulative emission will be observed from the abundance of this isotope in the interstellar medium. This results from the balance of decay and the production from all kinds of sources. In these cases of long-lived isotopes such as 26Al and 60Fe, gamma-rays from radioisotopes also should provide insights into the state of the interstellar medium which would be reflected in the measured line position and shape due to the Doppler effect. High-resolution solid state detectors in space, e.g. RHESSI (Lin et al., 2003), and INTEGRAL/SPI (Vedrenne et al., 2003; Roques et al., 2003; Winkler et al., 2003) have added this new quality to the ‘‘astronomy with gamma-ray lines”: Fine spectral resolution allows to measure the Doppler shifts, which are proportional to the photon energy and amount to keV-sized effects for 100 km/s velocities. From the first generation of measurements (HEAO-C, SMM, COMPTEL; see review by Prantzos and Diehl, 1996) we have * Corresponding author. Tel.: +49 89 30000 3850. E-mail address: [email protected] (R. Diehl). 1387-6473/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.newar.2008.06.024
learned that 26Al gamma-ray emission is extended over larger regions of the sky and, more precisely, distributed along the plane of the Galaxy with indications of enhancements in spiral-arm tangent directions and the Cygnus region (Diehl et al., 1995) (see Fig. 1). Since sources with individually-low contributions but high frequency of occurrence, such as novae or low-mass AGB stars, would be expected to result in a smooth spatial distribution of nucleosynthesis products in the Galaxy, this was taken as a main argument to favor massive stars as dominating sources. In a steady-state situation, massive-star contributions from core-collapse supernovae are indistinguishable from those during the WolfRayet phases of such stars (see Fig. 2). Therefore, attempts to disentangle these two nucleosynthesis sites have been based on theory arguments, using models for explosive nucleosynthesis or Wolf-Rayet stars to estimate the expected total yields. The suggested differences in yield dependencies from metallicity for supernovae and Wolf-Rayet stars, if combined with the observationally-indicated Galactic metallicity gradient, may provide an observational diagnostic, especially if combined with other observables from these types of sources (see Knödlseder, 1999). Deviations from a global steady state are expected for individual massive-star regions, as it is well known that massive stars preferentially are created in clusters as coeval groups of stars. In such stellar associations, depending on their age, Wolf-Rayet or core-collapse ejections of 26Al could dominate, for age intervals
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
3–5 and >5 Myrs, respectively (Plüschke et al., 2000, 2002; Cervino et al., 2000). The Cygnus region has been a prominent target of such studies, even though altogether nine OB associations are seen in overlapping projections. From population synthesis modeling it appears that Wolf-Rayet stars dominate 26Al production in this region (Plüschke et al., 2000; Cervino et al., 2000). INTEGRAL currently devotes a key program to this science objective, with altogether 10 Ms of exposure in the Cygnus region of the Galaxy. Other candidate regions with observationally-distinguished massive-star clusters are found in the Vela, Orion, and Sco-Cen regions (cmp. Fig. 1), and are also targets of specific 26Al studies. Here we summarize findings on the large-scale Galactic 26Al emission from the present INTEGRAL database and we report on the status and first results from spatially-resolved analysis.
2.
26
Al throughout the Galaxy
Various instruments have measured the large-scale 26Al emission from the inner regions of the Galaxy since its first detection in 1978 with the HEAO-C experiment (see review by Prantzos and Diehl, 1996). The measured fluxes, as normalized to the ‘‘inner Galaxy”, are roughly consistent and scatter around a value of 4 10 4 ph cm 2 s 1 for the central radian (which is a conventional reference and normalization, roughly covering the brightest regions in the sky, except for Cygnus). Imaging instruments appear to fall somewhat lower in their reported flux values: INTEGRAL/SPI measured a flux value of 3.3 (±0.4) 10 4 ph cm 2 s 1 (Diehl et al., 2006), while COMPTEL’s value was 2.8 (±0.4) 10 4 ph cm 2 s 1 (Plüschke et al., 2001). This may be an indication of either different
441
susceptibilities to instrumental background with a common feature near the 26Al line, or possibly an indication for extended-nearby or isotropic 26Al emission. In any case, depending on the adopted distribution of 26Al in 3-dimensional models of the Galactic sources, this translates into a total Galactic 26Al mass of 2.8 (±0.8) M . Here INTEGRAL/SPI added an argument from high-resolution spectroscopy of the 26Al line towards different directions in the inner Galaxy: The line shifts indicated by separating three different directions appeared consistent with what is expected from plausible models for spatial distribution of sources and rotational pattern in the Galaxy (Kretschmer et al., 2003). This supports the interpretation of the observed flux in terms of large-scale integrated emission along the full line of sight throughout the Galaxy, rather than possible systematic distortions from peculiar bright regions such as Cygnus at unknown but more nearby distances. It reaffirms earlier conclusions about the total Galactic amount of 26Al present in the current-day interstellar medium. Different astrophysical views of this 26Al mass measurement can be taken (see Figs. 3a,3b and discussion in caption). If massive stars are held responsible for this steady-state production, a combination of theoretical yields per mass of the star and a distribution function for numbers of stars per stellar-mass interval determines a steady-state rate of star formation, or, equivalently, a steadystate rate of core-collapse supernova events in the present-day (last few Myrs) Galaxy. From their balloon experiment measurement the GRIS team had reported an intrinsic line width of 5.4 keV (Naya et al., 1996), the celestial broadening of which would have corresponded to interstellar gas velocities of 500 km s 1. This was difficult to
Fig. 1. The Galaxy has been recognized as a diffuse source of gamma-rays at 1808.65 keV energy (Diehl et al., 1995) from the radioactive decay of 26Al (s 1 My), produced and ejected from nucleosynthesis sources throughout the Galaxy. The image presents the results from 9 years of observations with the COMPTEL telescope aboard NASA’s Compton Observatory (1991–2000; Plüschke et al., 2001). The spectrum shown in the upper right results from observations with the SPI Ge Spectrometer on ESA’s INTEGRAL Mission (since 2002) (Wang et al., in preparation; Diehl et al., 2006a). From the irregular appearance of the COMPTEL image and other arguments, it had been concluded that massive stars probably dominate the Galactic production of 26Al (see Prantzos and Diehl, 1996). From the narrow line shape observed with INTEGRAL, it is concluded that 26Al propagates in its source surroundings to obtain normal interstellar-medium velocities, on average (Diehl et al., 2006b).
442
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
Fig. 2. The final evolutionary phase of a massive star, shown in logarithmic time scale with the core collapse event at the rightmost end (adapted from A. Heger). This ‘‘Kippenhahn diagram” reflects the complex structure and energy production & transport within a star, the ordinate being the logarithmic mass coordinate with stellar core at the bottom and envelope at the top. Shaded regions denote nuclear energy production and neutrino energy losses, respectively, while hatched regions indicate where convection is responsible for energy transport. Mass loss sheds the outer envelope in later phases, carrying 26Al products from the early core hydrogen burning phase as it was mixed with the envelope. 26Al production which occurs later in the Si burning shell or during explosive O/Ne burning will be ejected in the terminal supernova explosion.
18
SFR [Moy-1]
15 12 9 6 3
G üs
Sm ith
e
ta l.1 97 Ta 8 lb ot te 19 n 8 et 0 al .1 98 Tu 2 rn er 19 M 8 ez 4 ge r1 98 M cK 7 ee va 19 M n 8 va cK 9 de n ee n de Be 19 n rg 89 Be h R rg & ad h Ta io 19 m Su 90 m pe an H rn n is ov to 19 ric a 91 R Su em pe na rn va C nt ov ap n s a pe de R lla n ec Be ro or rg et d h al & .1 M 9 cC 93 lu re M 19 cK Ti 94 Pa m ee m ge & es l1 W ,D 9 illi 94 ie am hl ,H s 19 ar 97 tm St an ah n le 19 r& 97 Pa lla 20 04 Ku nd t2 00 R 4 ee D d ie 2 00 hl et 5 al .2 00 6
0
Fig. 3a. Key parameters of our Galaxy, as derived from the determinations of the current Galaxy’s content of 26Al isotopes: The gamma-ray line measurement, combined with massive-star model yields and the stellar initial mass function yields a new method to determine the supernova rate from core collapses (above; rightmost data column). As indicated, uncertainties are large in estimating this quantity, since extragalactic as well as solar-vicinity-based estimations carry substantial systematic uncertainties from extrapolation towards the Galaxy. (from Diehl et al., 2006b).
understand (Chen et al., 1997) and is now clearly ruled out from results obtained with RHESSI (Smith, 2003) and SPI (Diehl et al., 2003). More data help to further constrain the line width, improving the 95% probability value from <2.8 keV (Diehl et al., 2003) to
1.3 keV (Wang et al., 2008; see Fig. 4; flattening of the probability distribution below 0.4 keV may indicate a true line broadening of a few tenths of a keV).It may be possible with further years of observation to obtain a width determination, rather than an upper limit.
443
R. Diehl et al. / New Astronomy Reviews 52 (2008) 440–444
350
30
Solar System Interstellar Grains 300
25
250 20 200
ISM γ rays
15
150 10 100
5
50
0 0 .0 -7
0 0 .0 -6
0 .0 -5
0 .0 -4
0 .0 -3
0 .0 -2
0 .0 -1
00 0.
Fig. 3b. Key parameters of our Galaxy, as derived from the determinations of the current Galaxy’s content of 26Al isotopes: The isotopic ratio 26Al/27Al for the current-day Galaxy derives from the gamma-ray measurement and the solar Al abundance attributed to the Galaxy’s ISM mass estimate (below; hatched area). This value is lower than the isotopic ratios found in extra-solar grains embedded in meteorites, which are assumed to condense out of nucleosynthetically-enriched ejecta near the source. The gamma-ray measurement also falls below the value determined from the variety of isochrones of solar-system meteorites, which formed 4.6 Gyrs ago. The metallicity (hence 27 Al abundance) presumably was lower then, and additionally it is believed that the solar system experienced a peculiar initial enrichment in 26Al. (from Diehl et al., 2006b).
Fig. 4. Constraints on celestial broadening of the 26Al line: The probability distribution is asymmetric, and only an upper limit can be determined. Over 4 years of INTEGRAL data, now yield a 95% limit below 1 keV, corresponding to Doppler velocities slightly above 100 km/s (Wang et al., in preparation).
3. Spatially-resolved
26
Al studies
INTEGRAL observations focused on the inner Galaxy during the initial mission years, extending exposure towards larger longitudes thereafter. Several key programs were selected which aim at largescale diffuse-emission or source population studies, and it is one of the mission goals to assemble a legacy database for Galactic-plane survey studies. The variation of 26Al emission along the plane of the Galaxy is a prominent science goal. Continuing the work performed in the inner Galaxy (Diehl et al., 2006a,b) which had obtained the hints for Doppler shifts according to the Galaxy’s large-scale rotation, we now have obtained first line centroid and width results for the bright ridge of 26Al emission (Wang et al., 2008) (cmp. Fig. 1).
Fig. 5 shows spectra obtained for different longitude intervals for different spatial bins. Regions of interest for localized studies have so far concentrated on groups of massive stars, which are well separated from the underlying disk population of the Galaxy; examples are the Cygnus region as well as the Vela and Orion regions. With deep exposure in the central directions of our Galaxy and the fourth quadrant, it now also appears promising to target the well-known nearby stellar groups of the Sco-Cen association. A hint of 26Al emission from these stars had been suggested from the COMPTEL image. This association has been suggested to be a prominent nearby laboratory for star formation triggered by successive generations of stars, due to their impact on nearby molecular clouds (Preibisch and Zinnecker, 1999), with latest star forming episodes being at the origin of the tenuous hot interstellar cavity around the Sun (e.g., Frisch, 1981 and follow-up work). SPI data from this region can barely be separated from the nearby bright inner-Galaxy region. Yet, first spectra clearly suggest 26Al emission, with enhanced brightness of 26Al emission north of the Galactic plane, when directly compared to its southern counterpart. From the localized spectra shown in Fig. 5, the Aquila region near longitudes 30° possibly is peculiar, and deserves further study. Additionally, a dedicated 3 Ms exposure of the Orion region has recently been completed and is being studied. In Orion, earlier stellar subgroups of the OB1 association have blown a cavity which extends towards us, and may be guiding the streaming of nucleosynthesis ejecta from currently-active nucleosynthesis subgroups. This is indicated in imaging studies of 26Al gamma-rays with COMPTEL (see Diehl, 2002). In summary, INTEGRAL’s spectral resolution and deep exposures along the plane of the Galaxy have significantly advanced 26Al studies in the Galaxy: large-scale results from COMPTEL imaging have been confirmed, and complemented by constraints on the average kinematic properties of the ISM surrounding 26Al source regions. More specific insights are at the horizon for localized emission
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Fig. 5. Spatially resolved determinations of the 26Al gamma-ray line parameters (Wang et al., in preparation). Although the statistical quality of these spectra is marginal, some indications for variations of line properties (centroid, width) can be seen. Upon deeper studies, this bears the potential to identify regions with peculiar kinematic properties such as bulk motion or increased line broadening due to enhanced ISM turbulence from, e.g., young wind-blown cavities.
regions, where both the source populations and ISM environments can be usefully constrained from other astronomical observations. Acknowledgements This contribution results from the collaborative work with the members of the INTEGRAL and SPI Teams, and fruitful discussions with many other colleagues; we are grateful in particular to Hubert Halloin and Andrew Strong at MPE, Pierre Jean, Jürgen Knödlseder, and Jean-Pierre Roques at CESR, and Dieter Hartmann at Clemson University. INTEGRAL is an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA. The SPI spectrometer has been completed under the responsibility and leadership of CNES/France, its anticoincidence system is supported by the German government through DLR grant 50.0G.9503.0. We acknowledge the support of INTEGRAL from ASI, CEA, CNES, DLR, ESA, INTA, NASA and OSTC.
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