The strength of a spectral absorption line is directly proportional to the number of absorbers in the proper ionization and excitation state multiplied by its transition probability. Thus transition probability (= oscillator strength) accuracies provide fundamental limits to the accuracies of stellar abundances. Fortunately, much progress has occurred in the past couple of decades in laboratory studies of atomic species that can be observed in stellar spectra. Several groups (notably those at Mons University, Lund University, and University of Wisconsin) have embarked on major efforts to provide reliable transition probabilities for large numbers of transitions of elements critical to studies of Galactic nucleosynthesis. Good general reviews and references to the lab work have been published by Wahlgren (2002, Phys. Scr., T100, 22) and Biemont & Quinet (2003, Phys. Scr., T105, 38). Transition probability data from the Mons University group may be found at their DREAM web site.

I have been collaborating with the University of Wisconsin atomic physics group (Jim Lawler, Betsy Den Hartog, and their associates) in lab/solar/stellar studies of neutron-capture and iron-group elements. The Wisconsin team combines their experimental data on radiative lifetimes and branching ratios to produce transition probabilities of up to nearly 1000 spectral lines per species, with typical accuracies of 5-10%. A description of their techniques can be found in Lawler et al. (2005, AIP Conf. Proc.., 771, 152). The transition data are then applied to determine new solar photospheric abundances and to re-examine abundances in selected metal-poor halo stars.

Chris Sneden

A snapshot summary of recent laboratory data for ionized samarium and gadolinium: These plots, taken from the Gd II study of Lawler et al. (2006), correlates approximate strength factors for lines with their wavelength occurrence. The line strengths increase with increasing abundance of the element in the Sun (εSun), increasing transition probability (gf), and decreasing atomic energy potential (χ) multiplied by the inverse temperature (θ). Please see the paper for a more detailed explanation of this figure. Click on picture to see larger image

Papers from the neutron-capture element series. These papers include laboratory data for one or more neutron-capture elements, and solar/stellar abundance analyses. Clicking on the species name will take you to the astro-ph preprint service copy of the paper (after acceptance for publication) or to the NASA Astrophysical Data System Abstract Service link to the article (when published).
La II: Lawler et al. (2001, ApJ, 556, 452)
Eu II: Lawler et al. (2001, ApJ, 563, 1075)
Tb II: Lawler et al. (2001, ApJS, 137, 341)
Nd II: Den Hartog et al. (2003, ApJS, 148, 543)
Ho II: Lawler et al. (2004, ApJ, 604, 850)
Pt I: Den Hartog et al. (2005, ApJ, 619, 639)
Sm II: Lawler et al. (2006, ApJS, 162, 227)
Gd II: Den Hartog et al. (2006, ApJS, 167, 292)
Hf II: Lawler et al. (2007, ApJS, 169, 120)
Er II: Lawler et al. (2008, ApJS, 178, 71)
Ce II: Lawler et al. (2009, ApJS, 182, 51)
Pr II, Dy II, Tm II, Yb II, Lu II: Sneden et al. (2009, ApJS, 182, 80)

Other neutron-capture species studies. I collaborated with another group on a study of ionized niobium.
Nb II: Nilsson et al. (2010, A&Ap, 511, 16)

Neutron-capture element identifications in metal-poor stars . These papers concentrate on stellar spectroscopy, but depend on critical assessment of lab transition data.
Cd I, Lu II, and Os II: Roederer et al. (2010, ApJ, 714, 123)
Te I: Roederer et al. (2012, ApJ, 747, 8)
many elements Roederer et al. (2012, ApJS, 203, 27)

Here are some other papers on neutron-capture species by the Wisconsin group. These contain laboratory data but no solar/stellar abundance analyses. Data from the Dy II, Tm II, and Lu II papers have been used in the Sneden et al. 2009 article cited above.
Ru I: Salih & Lawler (1985, J. Opt. Soc. Am. B,, 2, 422)
Rh I: Duquette & Lawler (1985, J. Opt. Soc. Am. B, 2, 1948)
Nb I and Hf I: Duquette et al. (1986, J. Quan. Spectr. Rad. Trans.,, 35, 281)
Mo I: Whaling et al. (1986, J. Quant. Spec. Rad. Trans., 36, 491)
Ta I and W I: Den Hartog et al. (1987, J. Opt. Soc. Am. B, 4, 48)
Tm II: Wickliffe & Lawler (1997, J. Opt. Soc. Am. B, 14, 737)
Dy II: Wickliffe et al. (2000, J. Quant. Spec. Rad. Trans., 66, 363)
Lu II: Fedchak et al. (2000, ApJ, 542, 1109)
Eu I, II, and III: Den Hartog et al. (2002, ApJS, 141, 255)
Ce I: Lawler et al. (2010, JPhB, 43, 085701)
Er I: Lawler et al. (2010, JPhB, 43, 235001)
Gd I: Lawler et al. (2013, JPhB, 44, 095001)
Nd I: Stockett et al. (2011, JPhB, 44, 235003)
Sm I: Lawler et al. (2013, JPhB, 46, 215004)

Fe-group species. In recent years the Wisconsin group has begun a new effort to determine accurate laboratory transition probabilities for neutral and ionized species of Fe-group elements (Z=21-30). Here are some papers from that ongoing work. Usually these papers also have solar/stellar abundance analyses. Only papers in our collaboration are listed here; see ADS for additional Fe-group papers by the Wisconsin team.
Cr I: Sobeck et al. (2007, ApJ, 667, 1267)
Mn I and II: Den Hartog et al. (2011, ApJS, 194, 35)
Ti I: Lawler et al. (2013, ApJS, 205, 11)
Ti II: Wood et al. (2013, ApJS, 208, 27)
Ni I: Wood et al. (2014, ApJS, 211, 20)
V II: Wood et al. (2014, ApJS, 214, 18)
V I: Lawler et al. (2014, ApJS, 215, 20)

Diatomic molecules. These are important contributors to cool-star spectra in many wavelength regions. For some time many species have lacked wavelength and transition probability data that are accurate enough to match their spectra obtained with modern astronomical echelle spectrographs. Lab molecular spectroscopists are giving renewed attention to the hydrides and CN. Here are studies with which I am involved directly or indirectly. Line lists in the style preferred by my spectrum synthesis code MOOG can be downloaded by clicking at the end of each line.
MgH: Hinkle et al. (2013, ApJS, 207, 26); linelist is here: MgH-MOOG
12C12C: Brooke et al. (2013, JQSRT, 124, 11); linelist is here: 12C12C-MOOG
12C13C: Ram et al. (2014, ApJS, 211, 5); linelist is here: 12C13C-MOOG
CN red: Sneden et al. (2014, ApJS, 214, 26); linelists are here: 12C14N-rMOOG; 13C14N-rMOOG; 12C15N-rMOOG;
CN violet: Sneden et al. (2014, ApJS, 214, 26); linelists are here: 12C14N-vMOOG; 13C14N-vMOOG; 12C15N-vMOOG;


All of the research described here has resulted from the collective efforts of large numbers of people, including my current and former students, and investigators at many astronomical institutions around the world. Their contributions are gratefully acknowledged. For many years I have benefited from grants by the U.S. National Science Foundation and the National Aeronautics and Space Administration. Their support has been crucial to my research.