Star Formation and Herbig Ae / Be Stars
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Star Formation and Herbig Ae / Be Stars Kaitlin Kratter University of Arizona The Missing Link, Santiago, Chile, April 2014 AB & SU Aurigae, Herbig, 1960 1960ApJS....4..337H Defining Herbig Ae/Be Stars • A or B spectral type • IR excess • Emission lines • Nebulosity Herbig, 1960 1960ApJS....4..337H Defining Herbig Ae/Be Stars • A or B spectral type • IR excess • Emission lines • Nebulosity table for theorists Herbig, 1960 1960ApJS....4..337H Defining Herbig Ae/Be Stars • A or B spectral type • IR excess • Emission lines • Nebulosity table for theorists table for observers Herbig, 1960 1960ApJS....4..337H Defining an object class, modestly Herbig 1960 Theoretical definition of “Intermediate” Mass Palla & Stahler 2005 • Upper limit: 8 • evolution: SN, winds no “PMS” • formation: no PMS, importance of radiation pressure, formation of HII region born radiative • Lower limit: 1.5-3 • evolution: significant mass loss • formation: multiplicity, radiative PMS Herbig Ae/Be’s don’t quite match up.. FIGURE 2. Revised PMS models for low- and intermediate-mass stars with protostellar initial condi- tions. Selected isochrones are also shown. (From [23]) found by Hayashi, Iben and others can be summarized as follows: • Protostars move from the forbidden zone to the border for quasi-static equilibrium: yes, but the HR diagram is much more reduced, especially for high luminosity stars. • PMS stars descend along the vertical Hayashi tracks: yes & not, since not all of them have an initial fully convective phase. Stars with mass > 3M begin the PMS phase as fully radiative objects. ∼ ⊙ • PMS stars join radiative tracks: yes, all stars go through a radiative phase, apart from stars less massive than 0.4 M that remain fully convective. ∼ ⊙ • PMS stars reach the Main Sequence: indeed, all of them do, but massive ones (M > 10 M ) are born on the main sequence. The main reason is that the Kelvin- Helmoltz∗ ∼ time⊙ scale is a sensitive function of mass and decreases quite rapidly for more massive objects, becoming shorter than the accretion time scale. Thus, such massive stars have no PMS phase at all and appear directly on the main sequence once they are optically revealed. These important properties can be appreciated in the HR diagram shown in Figure 2. A full account of the observational and theoretical properties of young stars and tests on the HR diagram can be found in [18] [19]. SOME OBSERVATIONAL TESTS As indicated before, the distribution of stars in the color-magnitude diagram of the cluster NGC 2264 provided the first evidence of the existence of a population of young stars in the contraction phase. Similar results were obtained by Merle Walker on the brighter members of other clusters, such as NGC 6530, IC 5146, and NGC 6611. Interestingly, the original measurements of NGC 2264 by Walker have been recently 26 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.196.209.100 On: Fri, 04 Apr 2014 21:35:21 Intermediate Mass Stars are in the Transition Zone • Luminosities: transition zone in stellar structure on the PMS (or the end of the PMS) • Accretion rates onto cores: transition zone between non-turbulent and turbulent initial conditions • Ionization: transition zone between irrelevance and HII region dominated feedback • Disks and Planets: the last (but easier to image) place for planets? Does transition imply a break in formation process? Do we need more than one mode of star formation: CMF vs IMF 2.2. IMF Universality in the Milky Way 2.2.1. Uncertainties in the IMF Determination From Re- solved Populations PossibleThe basic observational evidence method tofor derive a the single IMF con- sists of three steps. First, observers measure the luminosity function (LF) of a completemode? sample of stars that lie within some defined volume. Next, the LF is converted into a present day mass function (PDMF) using a mass-magnitude ings as a function of scale). This would allow observations log[ M / MOt ] relationship.-2 Finally, the0 PDMF is2 corrected4 for the star6 for- and theory to compare the entire dynamic range of turbulent mation history, stellar evolution, galactic structure, cluster fragmentation using all the information in the field rather dynamical evolution and binarity to obtain the individual- 0 than focusing on some, ultimately arbitrary, definition of a star IMF. None of these steps is straightforward,Hennebelle & and many “core” and a “cloud.” biases and systematic uncertainties Chabrier may betheory introduced dur- ing the process.‘last crossing’ A detailed discussion of these limitations at In summary, these theories provide a natural link be- -2 (predicted CMF) tween the CMF, GMC mass function, and star cluster mass low masses and in the substellar regime is given by Jeffries Fig. 1.— IMF functional forms proposed by various au- (2012) and Luhman (2012). function for a wide range of systems. This link derives thors from fits to Galactic stellar data. With the exception -4 When obtaining the LF, defining a complete sample of from the inherently scale free physics of turbulent fluctua- ‘frst crossing’ of the Salpeter slope, the curves are normalized such that log[ dN/dlogM ] stars for a given volume can be challenging. Studies of field (predicted tions plus gravity.the integral The over most mass robust is unity. features When of comparing these models with ob- stars from photometric surveys, either wide and relatively GMC MF) are the Salpeter-likeservational slope data, the (↵ normalization2.2 2.4 is), set which by the emerges total number shallow-6 (e.g. Bochanski et al., 2010) or narrow but deep ⇠ − (e.g. SchultheisChabrier et (mass al., x3) 2006), are magnitudeMilky Way limited GMCs and genericallyof from objects scale-free as shown processes, in Figure 2. and the log-normal- Kroupa (mass x3) like turnover, which emerges from the central limit theo- need to be corrected for the Malmquist bias. Indeed, when distances-4 are estimated-2 from0 photometric2 or spectroscopic4 6 rem. The location of the turnover, in any model where tur- log[ M / M ] analyticallyOffner different, et is toal represent 2014 the full mass range of parallax, the volume limited samplesonic inferred will be pol- bulent density fluctuations are important, is closely tied to the IMF as a series of power-laws, such as in Kroupa (2001, luted by more distant bright stars due to observational un- the sonic length,2002). which Above is0 the.2 M scale,, theR multiple-power-laws, below which ther- segments ⇠ Fig.certainties 3.— Core and mass intrinsic spectrum dispersion and GMC in the mass color-magnitude spectrum cal- mal or magneticand log-normal support with dominates a power-law over tail turbulence. at high masses Be- agreeculatedrelation. by ( ThisHopkins effect, can2012b) be fairly as a significant function and of change stellar the mass very well as shown by Figure 1. However, the form of the low this scale density fluctuations become rapidly damped andslope sonic of mass. the luminosity The grey function dotted by curve more than shows 10%. the Stud- CMF IMF at the low mass end is still relatively uncertain and sub- and cannot produce new cores. As a result, the location of inferredies of by nearby (Hennebelle stars, for and which Chabrier the distance, 2008), is determined while the the peak, andject the to ongoing rapidity debate of the (e.g., CMFThies turnover and Kroupa below2007, it, 2008; from trigonometric parallax, are not affected by this bias, Kroupa et al. 2013). The philosophical difference betweenlines indicate structures defined by the last (grey dashed) depend dramatically on the gas thermodynamics (Larson, but they are affected by the Lutz and Kelker (1973) bias, in these two forms reduces to whether one believes that star-and first crossing (dot-dashed) of the self-gravitating bar- 2005; Jappsen et al., 2005). Unlike what is often assumed, which the averaged measured parallax is larger than the true formation is a continuous process or whether there are dis-rier.parallax, The Chabrier if the parallax(2005) uncertainties and Kroupa exceed(2001) 10%. fits These to the however, the dependence on thermodynamics displayed by tinct physical processes that dominate in certain regimes,observedissues IMF reduce are the shown studies by within the grey 20 pc dashed to a completeness and light-grey turbulent modelssuch as and at the simulations stellar/sub-stellar with driven boundary. turbulence is dot-dashedlevel of lines,80% respectively. (Reid et al., 2007), Observed which Milky results Way in small GMCs ⇠ not the same asOther a Jeans-mass proposed functional dependence forms (see includeHennebelle a truncated ex- number statistics, especially at low masses. Cluster stud- ↵ β are indicated by the solid grey line with circles. ponential where dN M − (1 exp[( M/M )− ])dM and Chabrier 2009). The difference/ between− the− scalingp of ies do not have these problems since a complete sample can (de Marchi and3 Paresce1/2 , 2001;3/2 Parravano et al., 2011), the the Jeans mass, MJ cs/(⇢ G ), and the sonic mass, in principle be obtained from the complete cluster census. 2 Pareto-Levy' family of stable distributions, which includes However, secure membership cannot be assessed from pho- Ms vturb Rs/G, can be dramatic when extrapolating to ' the Gaussian (Cartwright and Whitworth, 2012), and a log-andtometry the formation alone and of proper cores. motion The measurements most important as well assump- as extreme regionsnormal such joined as with starbursts power laws where at both the low turbulent and high ve- masses tionsspectroscopic in these simulations follow-up are are theoften explicit necessary. treatment In addition, of self- locity, vturb(Maschberger, is large (Hopkins, 2013). It, 2013c). is usually not Consequently, possible to discrim- a gravitythe remaining and turbulent contamination driving, since by field these stars two may processes be large if lie better understandinginate between of these the role forms of on thermodynamics the basis of observations in ex- due cluster properties such as age and proper motion are similar at the very heart of the analytic models described above.