Beryllium A Cosmic Chronometer? Carolyn Peruta Mentor: Ann Boesgaard. 1. Introduction

Transcription

1 Beryllium A Cosmic Chronometer? Carolyn Peruta Mentor: Ann Boesgaard 1. Introduction Beryllium, the third lightest element in the universe, has the potential to unlock many of the mysteries surrounding Big Bang nucleosynthesis, Cosmic Ray theory, Galactic chemical evolution, stellar evolution, and stellar interiors. While Big Bang nucleosynthesis is responsible for the creation of the lightest elements, H, He, and Li, it did not last long enough to create Be. Heavier elements, including Be, are formed by nuclear fusion reactions inside stars, however Be is destroyed as it moves further out in the star. With these two production mechanisms ruled out, only one widely accepted method is left, spallation. The single stable isotope, Be-9, is created by this spallation wherein high energy cosmic rays (~150 MeV) interact with elements such as C, N, and O in the interstellar gas to produce lighter isotopes. This concept of spallation invented by Reeves, Fowler, & Hoyle (1970) with details described by Meneguzzi, Audouze, & Reeves (1971) is responsible for the presence of Be in the surface layers of stars seen as absorption lines in stellar spectra. This unique process by which Beryllium is formed is the key to understanding the aforementioned topics in astronomy today. It is still not certain whether spallation occurs within the vicinity of supernovae where C, N, O is excited into the interstellar gas or uniformly by high energy protons bombarding C, N, O in the interstellar gas. The evolution of Be could be a local process or a global process. If global, the instantaneous abundance can be expected to be characterized by a scatter around the mean value significantly smaller than for Fe or O (Pasquini et al. 2004). Beryllium would therefore be a more reliable chronometer than [Fe/H] or [O/H]. In this paper, I present Be abundances from 20 metal poor stars ([Fe/H] < -1.5). The stars were observed with high resolution/high S/N spectroscopy and abundances were determined by fitting synthesized spectra. These abundances are plotted against [Fe/H] and combined with results from previous Be studies to confirm whether or not there is an intrinsic spread in Be at low metallicities. If there is a spread, we can infer the most likely mechanism for Be formation is spallation in the vicinity of supernovae and therefore A(Be) is not a good chronometer. This study shows that there is a spread around [Fe/H] = -1.5 and -2.5 with a typical error of 0.10 dex for all Be abundances. 2. Observations Observations of Be in 20 metal poor stars were made on six different observing runs with the Keck HIRES spectrometer (Vogt et al. 1994). Abundances are determined using the Be II resonance lines at and in the ultraviolet spectral region accessible from the ground. To obtain measurements at these wavelengths we needed high resolution, high S/N light gathering power that only Keck I with HIRES can provide. The spectrometer has a quantum efficiency of 94% of the Be II wavelength range and spectral resolution of 45,000 with an effective dispersion of 0.22 pix -1. To obtain high S/N with minimal cosmic ray hits, the exposure times were kept to minutes and co-added for the dimmest stars. The S/N ranges from 25 to 129 with a median of 93 and mode of 97. On each run, 1 to 3 Th-Ar comparison spectra were taken along with several flat field exposures and bias frames to reduce the data. The Th-Ar comparison spectra from each night were used to determine the dispersion solution

2 from low-order Legendre polynomial fits to hundreds of lines. The data reduction was performed using IRAF routines. Stellar parameters were obtained in a consistent and careful manner. More details will be found in the Data and Analysis section. The observed stars are listed in Table 1. Table 1: Observations Star R.A. Dec S/N Exp Date BD Nov 7, 04 HD Nov 7, 04 G Nov 7, 04 G May 05 BD Sep 04 G Sep 04 G Nov 18, 04 BD Nov 7, 04 BD Jan 05 BD Mar 05 BD Nov 18, 04 BD Nov 7, 04 BD Nov 7, 04 HD Nov 18, 04 HD Sep 04 BD Sep 04 HD Nov 18, 04 BD Nov 7, 04 BD Nov 7, 04 BD Nov 7, Data and Analysis 3.1 Stellar Parameters A critical step in obtaining beryllium abundances is determining the required parameters to make a synthetic spectrum. The four necessary parameters needed to construct an accurate model are: effective temperature, metallicity, surface gravity, and microturbulent velocity. For metal poor stars near the turnoff point on the main sequence, the microturbulent velocity is assumed to be a constant 1.5 km/s (Magain 1989). Surface gravity was determined using the Y 2 Isochrones (Demarque et al. 2004). The remaining parameters were determined by means of an extensive literary search. Details for each parameter determination follow Determining T eff I have used three photometric colors as temperature indicators: (b-y), (V-K), and (R-I) on the Johnson scale. The (b-y) colors came mostly from Schuster and Nissen (Schuster & Nissen 1988, 1989a, 1989b) which were available for all but one of the stars. The (V-K) colors came from a variety of sources containing either (V-K) or individual values for V and K. All values

3 were averaged directly. The (R-I) values also came from a straight average of many sources. Final values for the three indices are listed in columns 2, 3, and 4 of table 2. Reddening corrections were made if necessary based on H-beta, c 1, and m 1. Corrections less than 2 Φ (0.020) were ignored. Any star that required (b-y) reddening corrections was also corrected in (V-K) and (R-I). The correction was obtained using the following equation: Where, 2 ( b! y) = m! c! ( "#) ( "#)! m ("#) o o o o! m c c ("#) m c 2 o o o o o (!") = # " m = m E( b # y) o 1 c = c E( b # y) o 1 E( b # y) = ( b # y) # ( b # y) Temperatures from each color index are obtained on the Carney scale (1983b) using the following relations: T = 5040 / ( ( b! y) ) eff T = 5040 / ( ( V! K) ) eff T = 5040 / ( ( R! I) ) eff o o o o The final temperature is a weighted average of 4:2:1 for T eff (b-y) o, T eff (V-K) o, and T eff (R-I) o respectively. Table 3 contains reddening corrections. Table 2: Photometry and Temperature Determination Star (b-y) (V-K) (R-I) (b-y) 0 (V-K) 0 (R-I) 0 T eff T eff T eff Final σ (b-y) 0 (V-K) 0 (R-I) 0 T eff BD HD G G BD G G BD BD BD BD BD BD HD HD BD HD BD BD BD Table 3: Reddening and Temperature Calculation for (b-y)

4 Star (b-y) c1 m1 Ref E(b-y) (b-y) 0 T eff (b-y) 0 BD GCPD, SN HD GCPD, SN G GCPD, SN G GCPD, SN BD G GCPD, SN G GCPD, SN BD GCPD, SN BD GCPD, SN BD GCPD, SN BD GCPD, SN BD GCPD, SN BD GCPD, SN HD GCPD, SN HD GCPD, SN BD GCPD, SN HD GCPD, SN BD GCPD, SN BD GCPD, SN BD GCPD, SN Determining [Fe/H] The [Fe/H] value for each star assumes a solar abundance of log N(Fe) = A search was conducted through the Vizier service for metallicities obtained from high resolution and high S/N spectra. Only articles with a stated solar abundance were used. Metallicities were then averaged to obtain the final value. Errors were obtained by calculating the standard deviation from the mean for each star. Metallicities for the stars and the sources are listed in table 4. A key for the abbreviated references follows in table 5. Metallicities without references were obtained from a VizieR Catalog search. Table 4:[Fe/H] Determination Star Hi Res [Fe/H] Reference σ BD BB85, RB88, MG89, PC HD Ryan, PC93, C G G No BD C G G BD HT88, FB BD Axer 0.10 BD Tom92, GT96, Spite96, FB BD

5 BD RB88, PC BD HD HD PC BD PC93, Spite HD N97, N BD Spite96, RB88, Tom92, PC93, GT96, FB BD Peter, RB88,MG89, Axer, Spite96, FB BD RB Table 5: Abbreviation Key Abbreviation Bib. Code. Reference BB A&A, 144, 343B Barbuy et. al. (1985) RB A&A, 192, 192R Rebolo et. al. (1988) MG A&A, 209, 211M Magain (1989) PC ApJ, 402, 699P Pilachowski et. al. (1993) Ryan 1998ApJ, 500, 398R Ryan et. al. (1998) C AJ, 114, 363C Carney et. al. (1997) No ApJ, 485, 320N Norris et. al. (1997) HT A&A, 199, 269H Hartmann et. al. (1988) FB AJ, F Fulbright et. al. (2000) Axer 1994A&A, 291, 895A Axer et. al. (1994) Tom AJ, 104, 1569 Tomkin et. al. (1992) GT A&A, 314, 191G Gratton et. al. (1996) Spite A&A, 302, 172S Spite et. al. (1996) N A&A, 326, 751N Nissen et. al. (1997) N A&A, 353, 722N Nissen et. al. (2000) Peter 1981ApJ, 244, 989P Peterson (1981) Determining Log g Beryllium abundances are heavily dependent on this parameter. An error due to an uncertainty of 0.5 in log g at an effective temperature of 6200 K is 0.18 dex for log g = 3.85 and 0.20 dex for log g = For comparison, an error of 100 K in effective temperature at 6200 K only creates an error of 0.04 dex for log g = 3.85 and 0.05 for log g =4.35. Values in the literature reflect poor estimates for log g across the board. To get more accurate values, I consulted the Y 2 Isochrones (Demarque et al. 2004). This method is self-consistent with the metallicity and effective temperature determinations. The stars positions were located in the log g versus effective temperature plane on the 10 Gyr isochrones. Each metallicity range had its own 10 Gyr isochrone to investigate. This was used along with the Schuster and Nissen s plot of (b-y) o versus c o to determine which of the double valued log g s were appropriate. Since this method involves interpolation between effective temperatures and their corresponding surface gravities, an error from the standard deviation from the mean of nearby values was adopted. All final parameters with their errors can be found in table 6.

6 Table 6: Parameter Summary Star Teff Φ [Fe/H] Φ Log g Φ (K) (dex) (dex) BD HD G G BD G G BD BD BD BD BD BD HD HD BD HD BD BD BD Beryllium Abundance Calculation The Beryllium abundances for all stars were determined using the program MOOG (Sneden 2973) modified in The stellar models used in this program were created by entering the parameters of table 6 into a Kurucz model. Oxygen enhancements have also been included since there are OH features in the Be blend. The initial oxygen enhancement was estimated from the relation presented in Boesgaard et al and adjusted visually until a good fit was reached. The relation, on the Carney temperature scale is as follows: [ O / Fe] =! ( ± )[ Fe / H] ( ± 050. ) Figure 1 shows the effects of enhancing oxygen for the fit. Figure 2 shows the difference between two different beryllium abundances of the same metallicity. Figure 3 shows the results of this study (black squares) plotted along with Be abundances from previous studies against metallicity. The obtained abundances are listed in table 7.

7 Fig. 1. Two separate synthesized models for BD which emphasizes the need for oxygen enhancement to get a better fit.

8 Fig. 2. Two fits displaying the differences between G with an abundance of and HD with an abundance of

9 Fig. 3. Be abundances vs. [Fe/H]. Open circles and filled triangles are from Stephens et al. (1997), Boesgaard et al. (1999, 2001), and Boesgaard (2000). Open squares are from Primas et al. (2000a, b). The open pentagons and triangles are from Santos et al (2002). The horizontal dotted line is the meteoric Be abundance, A(Be). The filled squares are from this paper.

10 Table 7: Results Star [Fe/H] σ A(Be) σ (dex) BD HD G G BD G G BD BD BD BD BD BD HD HD BD HD BD BD BD Conclusions Beryllium is the least difficult of the three light elements, Li, Be, and B because it has only one known source, Galactic Cosmic Ray spallation, only the isotope 9Be is stable, and it is less susceptible to depletion than Li in stellar interiors. Study of this element can lead to more knowledge of the galactic evolution, cosmic ray theory, stellar evolution, and Big Bang nucleosynthesis. The results of plotting the abundance of Be against metallicity show there is indeed a spread at [Fe/H] ~ -2.4 and [Fe/H] ~ There is also a large spread at [Fe/H] > -1 due to Be depletion in younger stars as explained in Boesgaard (2004). There is a systematic increase of Be with Fe as the galaxy evolves, however the correlation is not strong. There is a spread in the Be abundance of 0.8 at the metallicity -2.4 and 0.8 at This signifies that Be is more likely to have formed locally around supernovae rather than globally in the interstellar gas. Since Be is formed from spallation involving oxygen, it would be useful to plot A(Be) against [O/H]. Observations of oxygen at this low metallicity are harder to make and therefore only beryllium versus iron is plotted. Future work will be done to identify which stars in this group belong to the halo and which stars belong to the disk of our galaxy to further understand evolution of beryllium. To answer the question of the title; No, beryllium is not a great cosmic chronometer.

11 References Alonso, A., Arribas, S., & Martinez-Roger, C. 1994, A&AS, 107, , A&AS, 117, 227 Axer, M., Fuhrmann, K., & Gehren, T. 1994, A&A, 291, 895 Barbuy, B. 1988, A&A, 191, 121 Beers, T.C., Chiba, M., Yoshi, Y., Platais, I., Hanson, R. B., Fuchs, B., & Rossi, S. 2000, AH, 199, 2866 Beers, T. C., Rossi, S., Norris, J.E., Ryan, S. G., & Shefler, T. 1999, AJ, 117, 981 Boesgaard, A. M Origin and Evolution of the Elements. The Light Elements: Lithium, Beryllium, and Boron. Boesgaard A. M., Deliyannis, C. P., King, J.R., Ryan, S. G., Vogt, S. S., & Beers, T. C. 1999, AJ, 177, 1549 Boesgaard A. M., Stephens, A., & Deliyannis, C. P. 2005, ApJ, submitted Bonafacio, P. & Molaro, P. 1997, MNRAS, 285, 847. Burbidge, E. M., Burbidge, G. R., Fowler, W. A., & Hoyle, F. 1957, Reviews of Modern Physics, 29, 547 Carney, B. W. 1983a, AJ, 88, b, AJ 88, 610 Cayrel, R., Spite, M., Spite, F., Vangioni-Flam, E., Casse, M., & Audouze, J. 1999, A&A, 343, 923 Copenhagen University, O. & Royal Greenwich, O. 2001, VizieR Online Data Catalog, 1256, 0 Deliyannis, C. P. et al. 2005, submitted Eggen, O. J. 1979, ApJ, 229, 158 Esa,. 1997, VizieR Online Data Catalog, 1239, 0 Hartmann, K. & Gehren, T. 1988, A&A, 199, 269 Meneguzzi, M., Audouze, J, & Reeves, H. 1971, A&A, 15, 337 Norris, J. E., Peterson, R. C., & Beers, T. C. 1993, Apj, 415, 797

12 Peterson, R. C., 1981, ApJS, 45, 421 Pilachowski, C. A., Sneden, C., & Booth, J. 1993, ApJ, 407, 699 Rebolo, R., Molaro, P., & Beckman, J. E. 1988, A&A, 192, 192 Reeves, H., Fowler, W. A., & Hoyle, F. 1970, Nature, 226, 727 Ryan, S. G. & Deliyannis, C. P. 1988, ApJ, 500, 398 Schuster, W. J. & Nissen, P. E. 1988, A&AS, 73, a, A&A, 222, b, A&A, 221, 65 Sneden, C., Preston, G. W., & Cowan, J. J. 2003, ApJ 592, 504 Snider, S., Allend Prieto, C., von Hippel, T., Beers, T. C., Sneden C., Qu, Y., & Rossi, S. 2001, ApJ, 562, 528 Spite, M. Spite, F., & Millard, J. P. 1984, A&A, 141, 56 Tomkin, J., Lemke, M. Lambert, D. L., & Sneden, C. 1992, AJ, 104, 1568 Vogt, S. S., Allen, S. L., Bigelow, B. C., Bresee, L., Brown, B., Cantrall, T., Conrad, A., Couture, M., Delaney, C., Epps, H. W., Hilyard, D., Hilyard, D. F., Horn, E., Jern, N., Kanto, D., Keane, M. J., Kibrick, R. I., Lewis, J. W., Osborne, J., Pardeilhan, G. H., Pfister, T., Ricketts, T., Robinson, L. B., Stover, R. J., Tucker, D., Ward, J., & Wei, M. Z. 1994, in Proc. SPIE Instrumentation in Astronomy VIII, David L. Crawford; Eric R. Craine; Eds., Volume 2198, p. 362,