Rolf-Peter Kudritzki

Research Interests

- Extragalactic stellar astronomy

- Supergiant stars as tracers of chemical evolution and distances of galaxies

- Extragalactic distances

- Stellar spectroscopy

- Stellar atmospheres and radiative transfer

- Radiation driven winds of hot stars



I use quantitative stellar spectroscopy of blue supergiant stars (BSG) and red supergiant stars (RSG) like Rigel and Betelgeuse in Orion to determine the chemical evolution of star forming galaxies. I also use these objects to measure distances to galaxies and, thus, to contribute to a precision determination of the Hubble constant. In the following, I first discuss my work with BSG. After this, I will introduce the use of RSG as cosmic probes of the chemical evolution. Note that this is a close collaboration with many colleagues in Hawaii, the U.S. mainland, Chile and Europe: Miguel Urbaneja, Ben Davies, Maria Bergemann, Fabio Bresolin, Zach Gazak, Norbert Przybilla, Matt Hosek, Roberto Mendez, Wolfgang Gieren, Gregorz Pietrzynski, Chris Evans, Lee Patrick, Betrand Plez, Iting Ho, Jabran Zahid, Gabriel Dima, Ralf Bender, Stella Seitz, Andreas Burkert and many others.



Blue supergiants (BSG) in the spiral galaxy NGC 3621 at a distance of 20 million light years. The BSG can be easily identified as individual targets. With modern telescopes such as Keck on Mauna Kea and the ESO VLT at Cerro Paranal it is possible to take spectra of these objects with excellent signal-to-noise.



An ESO VLT/FORS spectrum of a BSG in NGC 3621 compared with spectra of BSG in the solar neighborhood. There is no difference in quality. This clearly demonstrates that precise spectral diagnostics of such stars in distant galaxies is feasible.



Observed mass-metallicity relationship of galaxies from spectroscopic studies of blue supergiant stars. The red square belongs to M81. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



The same relationship as above, but now overplotted with the average relationships obtained by Kewley and Ellison (2008) obtained from strong HII-region emssion lines. The 10 different overplotted relationships belong to different strong-line calibrations. Obviously, the calibration uncertainty of strong lines introduces enormous uncertainties for this method. These uncertainties are poorly understood. Calibration (1) corresponds to Tremonti et al. (2004) and (7), (8) represent two calibrations by Pettini and Pagel (2004). Our science goal is to use extensive blue supergiant spectroscopy to develop an improved calibration of HII-region emission line strong line methods. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



Wide field image of the Sculptor galaxy NGC 300 at 2 Mpc distance. We have studied the stellar content in much detail and have spectroscopically determined metallicity, metallicity gradient, extinction and distance from the blue supergiants marked by blue squares or circles.



NLTE model atmosphere spectra for one of the A supergiants in NGC 300. Different metallicities ranging from 1/20 solar to twice solar are assumed.



NLTE model atmosphere spectrum with a metallicity a factor of three smaller than solar compared with the observed spectrum of one of the A supergiant in NGC 300.



Stellar metallicity gradient in NGC 300. Note that this is the first determination of such a gradient based on the abundances of iron group elements. See Kudritzki, Urbaneja, Bresolin et al., 2008, ApJ 681, 269.



BSG as precision extragalactic distance indicators. We use a relationship between absolute bolometric magnitude and stellar gravity log g and effective temperature Teff, which was predicted by Kudritzki, Bresolin and Przybilla, 2003, ApJ 582, L83. The figure shows this flux-weighted gravity - luminosity relationship of blue supergiants in galaxies in the Local Group and beyond. With gravities g and effectiv temperatures T determined from the spectrum this relationship can be used to determine precise extragalactic distances. Note that spectroscopy of the supergiant stars provides an independent determination of interstellar reddening and extinction. The method is, thus, free from uncertainties introduced by inadequate estimates of reddening. Details and theoretical background of the FGLR are described in Kudritzki, Urbaneja, Bresolin et al., 2008, ApJ 681, 269. A stellar evolution discussion of the FGLR is given in Meynet, Kudritzki, Georgy, 2015, Astron. Astrophys., submitted (see preprints).



Flux-weighted gravity - luminosity relationship of BSG in all galaxies studied so far out to a distance of 7.6 Mpc. See Kudritzki, Urbaneja, Gazak et al., 2013, ApJ Letters 779, L20 or Hosek, Kudritzki, Bresolin et al., 2014, ApJ 785, 151 for further details.



Blue supergiant stars in the disk of the spiral galaxy M81 at 3.5 Mpc. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



M81 at 3.5 Mpc. Left: Selection of blue supergiant targets from the color-magnitude diagram. Right: Location of selected targets within M81. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.





Metallicity of blue supergiants in the disk of M81. Metallicity determination of object C20 as an example. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



Metallicity of blue supergiants in the disk of M81 as a function of galactocentric distance. We find slight super-solar metallicity in the center and a very shallow gradient. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



Metallicity of blue supergiants in the disk of M81 compared with planetary nebulae (PNe data from Stanghellini et al. 2010. The lower PN metallicity indicates strong chemical evolution of the disk over the last 5 Gyrs. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.





The Hertzsprung-Russell diagram (top) and the (log g, log Teff)-diagram of blue supergiant stars in M81. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



The distance to M81 from the observed FGLR compared to the Kudritzki et al. (2008) calibration. A distance modulus of m-M = 27.7 pm 0.1 mag is obtained. See Kudritzki, Urbaneja, Gazak et al., 2012, ApJ 747, 15.



A study of BSG in NGC 3621 at 6.5 Mpc



Hubble Space Telescope ACS images of BSG in NGC 3621. The circles correspond to 1 arcsec. Note that these objects are so bright that they shine out anything else. The fainter objects in these stamp images are more than four magnitudes fainter. Their brightness corresponds to Cepheid stars. This is one reason why BSG are potentially better distance indicators than Cepheids. Below we show spectral fits of the metal lines in the spectra of two BSG.







Examples of the model atmosphere fits of metal lines for 2 NGC 3621 targets. Note that we determine true metallicity not only oxygen abundance, as one does with HII regions. See Kudritzki, Urbaneja, Bresolin et al. 2014, ApJ 788, 56 for details.



Determination of the distance to NGC 3621 using the FGLR-method. See Kudritzki, Urbaneja, Bresolin et al. 2014, ApJ 788, 56 for details.



The "maser galaxy" NGC 4258 at 7.6 Mpc, an important anchor point of the extragalactic distance scale.



A BSG in the "maser galaxy" NGC 4258 and preparations for a spectroscopic study using the Keck 1 telescope on Mauna Kea and the LRIS spectrograph. See Kudritzki, Urbaneja, Gazak et al., 2013, ApJ Letters 779, L20 for details .



A BSG determination of stellar metallicity in the "maser galaxy" NGC 4258. This is fundamental for using this galaxy as an extragalactic distance scale anchor point. See Kudritzki, Urbaneja, Gazak et al., 2013, ApJ Letters 779, L20 for details .



Now I turn to red supergiants (RSG) as Betelgeuse in Orion. These are the brightest stars in the universe at infrared light. Together with my colleagues Ben Davies and Zach Gazak I have developed a method to use low resolution spectra in the near-infrared J-band to determine their chemical composition. In this way, we can use RSG as cosmic abundance probes in distant galaxies.



We tested the method in the nearby massive star association Per OB1 which comprises the massive double star cluster h and chi Persei and contains many RSG.



Spectral fit of near-IR J-band spectra of RSG in Per OB1. The metallicity obtained is similar to the sun and agrees with the metallicity obtained from massive hot stars of spectral type B and A and also BSG. For details see Gazak, Davies, Kudritzki et al., 2014, ApJ 788, 58. The spectral analysis is based on detailed non-LTE radiative transfer calculations which are described in the papers by Bergemann, Kudritzki et al., 2012, 2013, 2015 published in ApJ.



The multi-IFU spectrograph KMOS at the ESO VLT is an ideal instrument for our extragalactic RSG studies. We have done a pilot study in the spiral galaxy NGC 300 at 2 Mpc, for which we have also done an investigation of BSG.



Selection of RSG in NGC 300 from color magnitude diagrams.



Location of the RSG studied with KMOS/VLT in NGC 300. The size of a KMOS field is indicated by the grey circle. The rectangles indicate HST fields.



ESO VLT KMOS spectra of RSG in NGC 300 and spectral fits with Non-LTE calaculations. For details see Gazak, Kudritzki, Davies et al., 2015, ApJ, submitted (see preprints).



Stellar metallicities as a function of galactocentric distance in NGC 300. Blue symbols are the BSG from the study by Kudritzki et al., 2008, shown above. The black symbols are the new results from near IR J-band spectroscopy of RSG. The two regression curves are almost identical and indicate excellent agreement. This cleary demonstrates the accuracy and power of extragalactic stellar astronomy. This research field has now truly matured to become a precision tool to investigate the chemical evolution of galaxies.



The near IR cryogenic multi-object spectrograph MOSFIRE at the Keck telescope on Mauna Kea is the other instrument for our extragalactic RSG chemical evolution studies.



First J-band spectra of RSG in the Andromeda galaxy M31 obtained with Keck/MOSFIRE and first determinations of metallicities.



Super Star Clusters are another outstanding tool for chemical evolution studies of galaxies



The J-band light of SSCs is dominated by the red supergiants. 95% of the J-band light comes from RSGs as soon as the clusters are older than 7 million years.



A simulation of the spectral energy distribution of a young SSC and a SSC at the age of 15 million years. For clusters older than 7 million years the IR part of the SSC spectral energy distribution is dominated by RSG. See Gazak, Davies, Bastian, Kudritzki et al., 2014, ApJ 787, 142.



A simulation of the J-band spectrum of a SSC at the age of 15 million years. The contribution from RSGs clearly dominates. This is the combined light of fifty to hundred RSGs for a cluster with hundred thousand solar masses. Thus, SSCs are tremendously bright and can be studied specroscopically at enormous distances. See Gazak, Davies, Bastian, Kudritzki et al., 2014, ApJ 787, 142 and Gazak, Davies, Kudritzki et al., 2014, ApJ 788, 58 for details.



Using the spectra of RSG in Per OB1 of our previous study to simulate a an integrated SSC spectrum. The analysis gives the same metallicity as for the individual RSG in Per OB1. This demonstrates that the analysis of integrated SScC spectra is an accurate and p[owerful tool. From Gazak, Davies, Kudritzki et al., 2014, ApJ 788, 58.



A J-band pilot study of SSCs in M83 and NGC 6946 published in Gazak, Davies, Bastian, Kudritzki et al., 2014, ApJ 787, 142.



SSCs in the Antennae galaxies at 20 Mpc distance observed with MOSFIRE at the Keck 1 telescope.



Keck Mosfire J-band spectra of SSCs in the Antennae galaxies at 20 Mpc and a first determination of metallicity.



The enormous potential of J-band RSG spectroscopy reaching far beyond the Coma cluster of galaxies.

Some older theoretical work ....
  • Winds from Hot Stars. An Introduction
  • Winds, ionizing fluxes and spectra of the first generation of very massive stars in the early universe.
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  • Last revision: March 21, 2015