arXiv:astro-ph/0509264v1 9 Sep 2005 IL YTEGNRU IACA UPR FTHE OF SUPPORT FOUNDATION. FINANCIAL KECK GENEROUS W.M. THE POS- BY MADE SIBLE WAS OBSERVATORY SPACE THE AND ADMINISTRATION. AERONAUTICS NATIONAL CAL- THE OF AND CALIFORNIA UNIVERSITY IFORNIA THE THE TECHNOLOGY, OF AMONG INSTITUTE PARTNERSHIP SCIENTIFIC AS OPERATED A IS been OBSERVATORY, WHICH KECK has W.M. THE it Accordingly, ∗ interac- neighbors. more nearby gravitational them with from make tions accelerations lower to members whose susceptible smallest interac- association, the These an affect of growth. predominantly its would further stunting accret- tions preventing and heavily reservoir, accretion a mass its separate from protostar can ing encoun- protostars gravitational between of that al. indicate ters simulations et studies (Bate These environment N-body arguments cluster 2003). small and analytic a in formation through 2001) Clarke both & this suggested (Reipurth process; formation a been star the as has in protostars factor critical accreting possible between interactions namical process. formation existing star within the low results of of the models kinematics interpret the and investigate protostars we mass study observational this direct In to popula- inaccessible study. state protostellar largely spectrum) dynamical been flat has the and tions of I process. (Class diagnosis formation young a star of the however, for now, interest Until crucial of is tions 550 etl,W 98195 WA Seattle, 351580, A94035-1000 CA ail 0,L eea Chile Serena, La Astron in 603, Research Casilla for Universities of Association Center, tet abig,M 02138 MA Cambridge, Street, H AAPEETDHRI EEOTIE AT OBTAINED WERE HEREIN PRESENTED DATA THE 3 4 5 2 dy- identified recently have investigations Theoretical popula- protostellar embedded of state dynamical The rpittpstuigL using typeset 2018 Preprint 10, November DRAFT AAAe eerhCne,Mi tp256 oetField, Moffett 245-6, Stop Mail Center, Research Ames NASA rsn drs:Gmn bevtr,Suhr Operations Southern Observatory, Gemini Address: Present avr-mtsna etrfrAtohsc,6 Garden 60 Astrophysics, for Center Harvard-Smithsonian nvriyo ahntn eateto srnm,Box Astronomy, of Department Washington, of University H AILVLCT ITIUINO LS N FLAT-SPECTR AND I CLASS OF DISTRIBUTION VELOCITY RADIAL THE fyuhadaceinsc sH as such accretion and youth of ailvlct ulesmyrpeetpootla spectroscopic headings: protostellar represent Subject may outliers velocity radial eaiet h oa oeua a.Fu ftepootr analyze protostars infl the slightly molecular of for Four the allow gas. still 3 molecular of could local potential the dispersion to velocity gravitational relative protostellar the the on by limit dominated and linked ota ftelclC a,w r bet osri h n dimensio one the constrain to to able objects are spectrum we flat gas, and CO I local r Class the the of Comparing of clouds. that molecular to parental within buried populations Auriga, eoiydseso scnitn ihtevlct iprin fsurr of dispersions velocity be the to with consistent is dispersion velocity σ eaayerda eoiisfrasml f3 ls n a spectru flat and I Class 31 of sample a for velocities radial analyze We 75k sec km (7.5 ∼ ρ ei .Covey R. Kevin . msec km 1.4 piciadSresfreiec ftegoa yaia tt o state dynamical global the of evidence for Serpens and Ophiuchi 1. A INTRODUCTION T E − tl mltajv 11/26/04 v. emulateapj style X 1 wyfo h eta oa Ogsvlct hl hwn spectros showing while velocity gas CO local central the from away ) nrrdsas—sasfrain—saslwms :pre–ma — stars:low-mass — stars:formation — infrared:stars tr:kinematics stars: − 1 ugsigta h oin fpootr n oa Ogsaed are gas CO local and protostars of motions the that suggesting , 2 hmsP Greene P. Thomas , 2 msin IBr HI emission, RF oebr1,2018 10, November DRAFT ∼ m,Inc., omy, . msec km 2.5 ABSTRACT 3 rgW Doppmann W. Greg , − γ rmyugbondaf niae naceinrt of rate accretion an indicates M dwarfs brown young from ffc h trfrainpoes hsitiini but- interactions is which intuition to This degree process. the formation for star gas the radial proxy molecular affect protostellar a local the the protostars as of in the serves that increase above whence an dispersion from velocity short, gas In molecular that formed. to local pro- will similar the dispersion influence formation velocity of little star protostellar have a a effects in while result dynamical which gas, in molecular that cess above local protostars the the the of of inflate dispersion will velocity protostars intrinsic dy- between significant expect interactions We namical in formation. ejection star of ongoing signature of sites kinematic the measure directly interactions. to by disturbances masses, prior lower of expect to result might accretion a Tauri as we T possibly than scaling weaker brown linearly ac- isolated be from that strongly could indicate be accretion may to dwarf results appear These sample creting. their in dwarfs brown ueol ta.(03 n htaayi of However, siz- analysis that a population. find produce (2003) dwarf to al. brown et not the Muzerolle enough of rare suscepti- fraction most or able be disruption) should to which brown ble objects of very reservoirs either (the accretion seem must dwarfs the mechanism would disrupt ejection dwarfs completely possible not brown any mass young suggest steady of to isolated predominance in The ob- 2003; accretion 2003). al. been al. et have (Jayawardhana et al. well Luhman accretion as et spectra active Muench their in of 1999; served signatures of al. and et indicative 2001) (Wilking excesses infrared disks have formation possess dwarfs circumstellar star brown to Young larger observed the been mixed. are of however, context in process, the role in regime. large mechanism substellar a the play in spectrum may mass system the from shaping multiple ejections unstable or interactions an dynamical that suggested 1 msin rKbn otnu eln.These veiling. continuum band K or emission, ˙ rblw hsuprlmtt h protostellar the to limit upper This below. or oedrc rb fteeeto aaimwudbe would paradigm ejection the of probe direct more A bevtoa le oteiprac fteejection the of importance the to clues Observational ∝ M − 2 ewe n 0.05 and 1 between iaiso jce lse members. cluster ejected or binaries pert aevlcte oethan more velocities have to appear d udn Ogswihw measure we which gas CO ounding 3,4 da eoiyo ahprotostar each of velocity adial hre .Lada J. Charles , a ailvlct dispersion velocity radial nal lu.Hwvr h upper the However, cloud. tdmtoso protostars of motions ated xrml on stellar young extremely f rtsasi Taurus- in protostars m MPROTOSTARS UM M oi indicators copic ⊙ nsqec — in-sequence n htol f13 of 3 only that and , 5 ynamically Hα emission ∗ 2 Covey et al. tressed by the results of numerical simulations of the star evolved T Tauri stars in the same clouds. Duchˆene et al. formation process, which show stellar interactions can in- (2004) also find that Class 0 and I sources with millime- crease the protostellar velocity dispersion by a factor of ter emission from protostellar envelopes have higher com- three above the velocity dispersion of the molecular cloud panion star fractions (38%) than Class I/II/III objects from which they formed (Bate et al. 2003). Precise mea- without significant millimeter envelope emission (22%). surements, however, are required to test this model, as These results suggest the multiplicity fraction of stars N-body simulations predict velocity dispersions even in may decline through the pre-main sequence phase from such interacting environments at the 1-2 km sec−1 level. high fractions in the most heavily embedded phases to The dynamic state of protostellar populations is also lower fractions on the main sequence. The observa- interesting in the context of cluster formation and dis- tions analyzed here are sensitive to binaries with large ruption. The majority of young, forming stars in the orbital velocities, sampling binaries with much smaller Milky Way are to be found in cluster environments separations than the studies of Haisch et al. (2004) and (Lada & Lada 2003). However, the stellar population Duchˆene et al. (2004), useful for probing possible effects of the Milky Way is dominated not by clusters of orbital evolution between the protostellar, T Tauri and but by the diffuse field population making up the thin main sequence phases. and thick disk populations, presumably from stars which In this study, we investigate the dynamical motions formed in now dissipated clusters (Majewski 1993). Ex- of Class I and flat spectrum protostars by studying the plosive gas removal is thought to efficiently disrupt the dispersion of protostars about the local majority of young clusters on timescales of 5-10 Myrs. CO gas velocity. In §2 we describe the observations used Those clusters which do survive the dispersal of their to derive radial velocities for Class I and flat spectrum parental gas cloud eventually fall prey to evaporation protostars, as well as the observations which allow us to and tidal heating, processes which operate on longer determine the systemic velocity of the CO gas associ- timescales of 108−9 (Lada & Lada 2003). These ated with each star formation region. Section 3 contains effects are thought to leave measurable kinematic sig- our analysis of the resultant radial velocity dispersion natures on the populations they effect. Mass loss from for Class I and flat spectrum sources, while §4 constrains clusters, for example, is thought to result in a decreased the possible source of four prominent protostellar radial velocity dispersion for the remaining bound members; velocity outliers. Section 5 discusses the implications of the ratio of the initial and final velocity dispersions are these results in the context of the standard picture of correlated with the overall star formation efficiency in the low mass star formation, and a summary of this work is cluster (Mathieu 1983). Measuring the dynamic state of contained in §6. protostellar populations can provide insight into mech- 2. anisms of cluster disruption by providing constraints on OBSERVATIONS & VELOCITY DETERMINATIONS the initial velocity dispersion of newly formed stars for This study utilizes data from two distinct sources; the numerical simulations of cluster disruption, as well as by near IR spectral survey of Class I and flat spectrum pro- probing cluster evolution with a comparison of protostel- tostars presented by Doppmann et al. (2005) (hereafter lar populations to more evolved bound clusters. D05), and the CO survey of the Milky Way described The velocity dispersion of protostars in embedded clus- by Dame et al. (2001). Though we refer the reader to ters has not yet been observationally determined. The ra- the publications cited above for a full and complete dis- dial velocity dispersion has been studied for more evolved cussion of the observations conducted by each survey, T Tauri stars beginning in the work of Herbig (1977) and we provide below a brief summary of those observational continuing through the work of Jones & Herbig (1979), details most relevant to the work presented here. Hartmann et al. (1986) and Dubath et al. (1996). These studies find no difference above the errors (typically ∼ 2.1. Protostellar Observations and Derived Radial 0.5 km/sec) between the mean stellar velocity and the Velocities central velocity of the local gas, and measure 1-D radial The protostellar radial velocities used in this study velocity dispersions of 2 km sec−1 or less. A study of this were derived using sprectra from the Class I and flat spec- type has not yet been performed for protostellar popula- trum survey conducted by D05. Class I and flat spectrum tions, primarily because the spectroscopic resolution and sources were observed by D05 over the course of 12 nights sensitivity needed to measure radial velocities at the 2-3 between May 2000 and June 2003 using the near infrared km sec−1 level has only recently become accessible in the echelle spectrograph NIRSPEC (McLean et al. 1998) on infrared where these very young, highly embedded and the 10 m Keck II telescope. Spectra were acquired with a reddened objects are easier to detect. 0.58” (4 pixel) wide slit (allowing R ∼ 17,000), utilizing a Radial velocity measurements also serve to constrain 1024x1024 pixel InSb detector array with the NIRSPEC- the fraction of stellar systems which are members of 7 blocking filter, under good (∼ 0.6”) seeing. The NIR- close multiple systems. Recent near infrared imaging SPEC gratings were configured such that 4 prominent surveys of Class I and flat spectrum sources in the sets of spectral lines could be simultaneously observed in Taurus, ρ Ophiuchi, & Serpens star formation regions non-continuous orders: 1) the 2.1066 µm Mg and 2.1099 (Haisch et al. 2004; Duchˆene et al. 2004) show compan- µm Al lines, 2) the 2.16609 µm HI γ line, 3) the 2.2062 ion star fractions of ∼ 20 − 25% for separations be- and 2.2090 µm Na doublet (blended with lines of Si and tween 300 and 2000 AU. The companion star frac- Sc) and the 2.2233 µm H2 line, and 4) the 2.2935 µm tion of embedded protostars is thus significantly ele- CO bandhead. We have selected for our analysis pro- vated above that detected for solar type main sequence tostars with detected absorption features located in the stars in the same range of separations (12%; Mathieu ρ Ophiuchus, Taurus-Auriga or Serpens star forming re- 1994; Duquennoy & Mayor 1991), as also found for more gions and observed by D05 in July 2001 or later, as the Protostellar Radial Velocities 3

May 2000 data presented by D05 has much larger wave- length calibration uncertainties. These criteria resulted TABLE 1 in a sample of 31 protostars suitable for analysis in this Radial Velocities of Class I and Flat Spectrum Protostars work. The set of MK spectral type standards observed Target name Vlsr CO Vlsr Vdiff CO-stellar dist by D05 have also been included in this analysis to provide (km/sec) (km/sec) (km/sec) (beam widths) estimates of observational uncertainties. 04016+2610 -10.42 6.83 -17.25 0.56 A discussion of the methods used to reduce spectral 04489+3042 -18.27 6.83 -25.10 0.61 data is presented in full by D05. In summary, all spec- 04295+2251 3.83 5.53 -1.70 0.48 tra were flat-fielded, cleaned, sky subtracted, extracted, Haro-6-28 3.64 5.53 -1.89 0.26 wavelength calibrated and corrected for telluric absorp- 04108+2803B 6.59 6.83 -0.24 0.43 DG-Tau 8.20 6.83 1.37 0.45 tion using standard IRAF packages and procedures. The GV-Tau S -6.22 6.18 -12.40 0.54 radial velocity of each star was determined through cross 04264+2433 6.72 6.18 0.54 0.40 correlation (using IRAFs FXCOR package) with spectral 04158+2805 5.47 7.48 -2.01 0.43 standards observed by D05 and with well measured radial 04181+2655 7.55 6.83 0.72 0.39 L1551-IRS5 7.89 6.83 1.06 0.55 velocities (Nidever et al. 2002). The targets examined in IRS43 3.01 3.74 -0.73 0.43 this work range in spectral type from late G to mid M; WL12 -0.61 3.48 -4.09 0.19 given the onset of significant continuum structure for ob- IRS67 3.26 4.78 -1.52 0.78 YLW16A 4.41 3.74 0.67 0.57 jects of types M0 and later, we divided the targets into 162636 3.97 3.22 0.75 0.42 pre and post M0 sub-samples for the purposes of cross- GY21 2.65 3.22 -0.57 0.83 correlation. Targets with spectral types earlier than M0 GY224 2.30 3.74 -1.44 0.82 were cross correlated with template spectra of HD166620 VSSG17 7.06 3.48 3.58 0.68 WL3 3.71 3.48 0.23 0.84 (K2), HD201091 (K5) and HD219134 (K3). Targets with WL17 4.88 3.48 1.40 0.89 spectral types of M0 or later were cross correlated with IRS51 6.30 3.74 2.56 0.12 template spectra of Gliese 806 (M2), Gliese 4281 (M6.5) GY197 6.52 3.48 3.04 0.70 and VB8 (M7). Cross correlation was performed on 3 GY91 2.91 3.48 -0.57 0.85 CRBR12 -4.60 3.22 -7.82 0.62 echelle orders containing Mg, Al, Na and CO absorption EC125 5.07 7.91 -2.84 0.92 features; each object was compared to all three applicable EC38 12.88 8.56 4.32 1.22 templates using all orders with detected absorption fea- EC91 5.66 7.91 -2.25 0.77 tures. A mean heliocentric velocity for each object was EC92 13.02 7.91 5.11 0.82 EC94 9.70 7.91 1.79 0.79 then found as the average of 9 independently derived ve- EC129 10.51 7.91 2.60 0.71 locity estimates (from 3 orders in each of 3 standards) in EC53 13.36 8.56 4.80 1.26 all but two cases. Disk emission in EC38’s CO order re- duced the number of independent velocity estimates to 6, while WL12 produced extremely poor velocity estimates via cross correlation, so we have adopted D05’s radial The radial velocity dispersion of the protostars in each velocity estimate for this object. The radial velocity of individual star forming region can be calculated directly these protostars with respect to the local standard of rest from the VLSRs reported in Table 1. As we have only (LSR) is presented in Table 1, after corrections for the observed ∼ 10 protostars in each star formation region, solar motion with respect to the LSR were applied using individual radial velocity dispersions for each star for- IRAFs RVCORRECT task. mation region would be very sparsely sampled, possi- To test the accuracy of the radial velocity determina- bly returning spurious results. To improve the statistical tion resulting from this procedure, we have compared our leverage of our analysis, we have chosen to investigate derived heliocentric radial velocities with reported values the motions of protostars relative to the central velocity for 15 spectral standards observed by D05 with accu- of the local molecular gas. Assuming that the dynamical rately determined radial velocities (errors < 2 km sec−1; behaviors in the Taurus-Auriga, Serpens and ρ Ophiuchi Mohanty & Basri 2003; Nidever et al. 2002; Gizis et al. star formation regions are equivalent to first order, study- 2002; de Medeiros & Mayor 1999). Figure 1 displays the ing protostellar velocity residuals will allow us to combine residuals obtained when the radial velocity reported in results derived from protostars in different star formation the literature is subtracted from our derived value. We regions, generating a snapshot of the dynamical state of assume that the spread of non-zero residuals in Figure newly formed stars. The dispersion of protostellar radial 1 is due primarily to observational error – that is, we velocities about the local gas velocity can also be com- believe Figure 1 represents the expected observed radial pared to the velocity dispersion of the gas itself, allowing velocity distribution for a population of stars with iden- investigations of the extent to which stars and gas re- tical radial velocities (σ = 0 km/sec) and observational main dynamically linked during the Class I/flat spectrum errors typical of the limits of the observations presented phase. This enhanced statistical power comes at the ex- by D05. A best fit guassian to this distribution, shown pense of our ability to investigate differences in dynamics in binned form in Figure 1 as a dashed histogram, re- between star formation regions. Additionally, if ejection sults in a gaussian width (σ) of 1.28 km sec−1. We thus and evaporation are already important dynamical mech- conservatively adopt 1.5 km sec−1 as an estimate of the anisms by the Class I/flat spectrum stage, we might ex- uncertainty in our velocity determinations for slowly ro- pect protostellar surveys to preferentially detect candi- tating objects with strong absorption features. dates whose motion has carried them to regions of the 2.2. cloud with lower optical depths, as viewed from Earth. Molecular Cloud Observations and Derived Radial This effect would be detected by a protostellar radial ve- Velocities locity distribution with a peak at lower (bluer) velocities 4 Covey et al.

Fig. 1.— Histogram display of residuals (solid line) when heliocentric Fig. 2.— Histogram display of protostar residual velocities (solid velocities derived from NIRSPEC spectra of spectral type standards are line; Vstellar -VCO ). The best fit gaussian to this distribution, binned −1 compared to accurate (errors ≤ 2 km sec ) determinations from the in the same manner, is shown as a dashed histogram. The mean and literature for the same stars. The best fit guassian to the histogram, σ of the gaussian fit are displayed as well; though the protostars ap- −1 binned in the same manner, is shown as the dotted line. The gaussian pear to have a wider velocity dispersion (σ ∼ 2.59 km sec ) than σ provides an indication of the level of accuracy with which we can the standards shown in Figure 1, the rotational broadening and ac- −1 assign velocities (σ ∼ 1.3 km sec ). cretion veiling present in protostellar spectral features lead to more uncertain velocity determinations. The expected magnitude of these effects match well the excess width of the protostellar velocity disper- than the associated CO gas, as we preferentially detect sion, suggesting we have yet to measure the true velocity dispersion of stars moving toward us and are less sensitive to objects protostellar populations (see Section 3). moving away from us, deeper into the cloud. maximum deviations up to 0.5 km sec−1. The central To determine the difference between the radial velocity CO velocity at the nearest sampled position to each pro- of each protostar and its local molecular gas, we relied on tostar is shown in the third column of Table 1, and the measurements from the CO survey of the Milky Way by difference between the CO velocity and the protostellar Dame et al. (2001). This survey homogeneously mapped velocity is shown in the fourth column. The fifth column all three star forming regions from which the protostars shows the distance between the protostellar position and studied here were selected, sampled with a 0.125◦ beam the center of the CO sampling position, in units of the (FWHM profile) and a grid spacing (measured in units of beam profile (FWHM). ◦ and ) of 0.25 in the Serpens 3. and ρ Ophiuchus molecular clouds, and 0.125◦ spacing THE PROTOSTELLAR RADIAL VELOCITY DISPERSION in Taurus. The central molecular gas velocity associated Given the high degree of homogeneity in both the CO with each protostar in the sample was measured from the and NIRSPEC surveys, we are able to examine the radial profile of the CO line at the nearest sampled position on velocity dispersion about the local CO systemic velocity the sky, usually within 1 beam-width of the protostel- of these protostars in bulk. Shown in Figure 2 as a solid lar position. CO linewidths were typically 1-2 km sec−1. line is a histogram of the residual protostellar velocities The central molecular gas velocity was found by bisect- about the central velocity of the local CO gas; a dashed ing the portion of the CO line which lies above 25% of line indicates the best fit gaussian to this distribution. A the peak CO emission at that location. This method comparison of the gaussian fits to the radial velocity dis- allows for a robust determination of the central cloud tributions in Figures 1 and 2 show the protostellar pop- velocity which is insensitive to the precise line profile ulation has a measured velocity dispersion almost twice of the gas, a desirable quality given the optically thick that measured for our set of standards (standards σ = and non-gaussian nature of many of the CO line profiles 1.28 km sec−1, protostellar σ = 2.59 km sec−1). we examined. Typical differences between line center The excess width of the protostellar velocity dispersion velocities given by this method as opposed to standard above that of the velocity dispersion of the radial veloc- gaussian fits to the CO profile are 0.2 km sec−1, with ity standards is likely due to factors which make the pro- Protostellar Radial Velocities 5 tostellar velocity determinations less reliable than those derived for the standards. The first of these factors is the shallower and broader spectral features detected in protostellar spectra. The presence of continuum veiling and moderate rotational velocities in these protostars re- sult in spectral features which are shallower and broader than features in standards of similar spectral types. The decreased strength of these features result in cross cor- relation functions whose central velocity is more uncer- tain, demonstrated by examining the standard deviation of velocities determined for the same object from differ- ent templates; radial velocities determined for standards from each of three appropriate templates show a median standard deviation of 0.8 km sec−1, while the median standard deviation of the velocities derived for the pro- tostellar sample is 1.2 km sec−1, 50% larger. Additional width may be introduced into the protostellar velocity dispersion given the uncertainty in the CO velocity to which each protostar is being compared – while our ob- servational uncertainty has been examined by comparing our derived velocities to known velocities with typical er- rors on the order of tens of meters per second, errors in the local CO velocity could be as large as 0.2 km sec−1. The combination of weaker protostellar features and uncertainty in the local CO velocity implies the obser- vational limits of the measured protostellar velocity dis- persion are ∼ 0.6 km sec−1 larger than of our standards, such at a zero velocity population would likely have a measured velocity dispersion of ∼ 1.9 km sec−1. This re- duces the excess detected width of the protostellar veloc- −1 ity dispersion to 0.7 km sec above the level of expected Fig. 3.— Comparison between the cumulative probability functions observational uncertainties. This excess width could be of the standard and protostellar radial velocity residuals. The proba- bility the two functions originated from the same parent distribution, explained by an intrinsic protostellar velocity dispersion as estimated by an application of the KS test, is shown to be 48%. of 1.8 km sec−1 added in quadrature to the estimated 1.9 km sec−1 observational uncertainties. It is equally stellar interactions on the dynamical state of protostars plausible, however, that measuring the width of the ve- must therefore lie below the ∼ 2.5 km sec−1 level, or locity dispersion with a gaussian fit to a population with must be dominant only on timescales longer than that of 31 members will allow additional statistical uncertainties the Class I/Flat Spectrum lifetime. due to sampling errors. 4. To test the statistical importance of any difference RADIAL VELOCITY OUTLIERS between the observed protostellar-CO velocity residuals Although the radial velocity distribution of the pro- and the observed radial velocity standard residuals, we tostars is consistent in bulk with that of the surround- have carried out a Kolmogorov-Smirnov (KS) test on the ing CO gas, we do detect four outliers (04016+2610, two sets of data. The test is illustrated in Figure 3, 04489+3042, GV Tau S & CRBR 12) separated from which compares the cumulative probability functions of the central velocity of the local gas by more than 7.5 the protostellar and standard residuals. The KS statistic km sec−1. Given our ±2 km sec−1 radial velocity un- measures the maximum difference between two cumula- certainty, these objects represent 3σ outliers from the tive distribution functions and can be used to calculate central velocity of the local CO gas. All four outliers the probability that two distributions were drawn from possess negative (blueward) velocity shifts with respect the same parent sample (Press et al. 1986). As seen in to the local CO gas and other protostars in the same Figure 3, the two distributions have a nearly 50% chance star forming region – the outliers possess anomalous ve- of being derived from the same parent population. locities, and are not outliers due to peculiar local CO We therefore find that the dispersion of protostellar ra- velocities. The three most significant outliers reside in dial velocities about the local CO gas velocity does not the Taurus-Auriga star forming region. These velocity differ significantly from the same quantity as calculated outliers also appear to be bona fide protostars, and not for a sample of standards with accurately known radial older stars along the line of sight to the cluster: all 4 velocities. Given our observational errors, this constrains radial velocity outliers were found by D04 to have rk ≥ the width of the radial velocity dispersion of Class I/flat 0.9, all but CRBR 12 show HI Br γ emission, and two −1 spectrum protostars to below ∼ 2.5 km sec . Within (04016+2610, GV Tau S) show H2 emission. the limits of our measurements, therefore, we find no evi- There are 3 plausible explanations for such extreme dence that protostars and their local gas inhabit different velocity discrepancies in the radial velocities of young dynamical environments – that is, at the ∼ 2.5 km sec−1 protostars: a) the protostars have had a close gravita- level, protostars and the local gas appear to be respond- tional encounter with another member of the cluster and ing to the same gravitational potential. Any effect of gained energy enough to be ejected from the cluster, b) 6 Covey et al.

TABLE 2 Maximum Orbital Periods for Possible Binaries

Target name Estimated M Maximum Binary Separation of Obs. (M⊙) (years) (AU) (MM-DD-YYYY) 04016+2610 1.6 2.08 2.40 11-04-2001 04489+3042 0.15 0.06 0.11 11-04-2001 GV-Tau S 1.6 5.55 4.62 11-06-2001 CRBR12 0.75 10.45 5.47 06-20-2003 the protostar is a member of a binary with a systemic ve- mass components – generating the same radial velocity locity equal to that of the local CO gas, but was observed offsets with lower mass companions require smaller or- at a time when its primary’s orbital velocity is directed bital separations and shorter orbital timescales. in large part along our observational line of sight, or c) Assuming a canonical age for these outliers of 1 Myrs, the velocities derived for these outliers have large uncer- the effective temperatures determined by D05 for the tainties that are poorly characterized by our analysis of system can be translated into a mass via the models the velocities derived for stars with well known radial of Baraffe et al. (1998). Table 2 displays the estimated velocities. masses derived from the Baraffe models for each of these The last two scenarios presented above can be eas- outliers – in our simple model of these outliers as binary ily confirmed or rejected with additional observations at systems, we adopt the mass estimate given in Table 2 for later epochs. It is plausible that the measurements re- each component of the binary. The two hottest veloc- ported here are significantly uncertain; three of these out- ity outliers, GV Tau S and 04016+2610, have tempera- liers have rotational velocities in excess of 25 km sec−1, tures ∼ 250 K hotter than the hottest stars computed by making cross correlation peaks more difficult to centroid, Baraffe et al., requiring a small extrapolation to extend and three of these sources have been observed in the the grid to derive their likely masses. optical by White & Hillenbrand (2004), who find radial With mass estimates for the binary system compo- velocities for these objects (epoch Dec. 1999) that are nents, we can now calculate the maximum timescale for consistent with other members of the Taurus star form- changes in the observed radial velocity if the velocity ing region. For these reasons, additional observations to separation from the CO velocity is due to a binary or- confirm the radial velocities reported here are necessary bit centered on the mean CO gas velocity. To calculate before any conclusions can be drawn as to the nature of this maximum orbital period, we assume the observed these outliers. However, to guide the interpretation of systems consist of equal mass, single line spectroscopic the resulting observations, we briefly examine below the binaries in circular, edge on orbits with orbital velocities possible ramifications of results of followup observations. equal to the velocity separation between the observed ve- If multiple followup observations show in each epoch locity and the local CO gas (shown in column 4 of Table a protostar with a radial velocity equal to that of the 1). Under these assumptions, the system’s orbital period local CO gas, it is safe to assume that these objects are is related to the masses of the stars by: outliers due to uncertainties in the analysis presented m3 P here (option c). Additionally, if the radial velocities of comp v3 2 = obs (1) these outliers are observed to change in a statistically (mobs + mcomp) 2πG significant fashion over the course of the follow up ob- servations, it would be likely that their velocity excesses where mobs and vobs indicate the estimated mass and de- result from motions due to binarity (option b). However, tected residual velocity of the observed component, and multiple followup observations showing radial velocities mcomp indicates the upper limit to the mass of any com- consistent with those reported here would not necessar- panion. Our assumption of equal mass binaries implies ily be proof that the stars in question are being ejected that mobs = mcomp. With the observed orbital velocity from the star forming region, as it is conceivable that the fixed, the assumptions of an equal mass binary in a circu- outliers could be members of a binary with a long orbital lar, edge on orbit with maximal projection of the orbital period. velocity along the radial velocity direction drive the de- To constrain the timescales over which a consistent ra- rived orbital period to the maximal end of the allowed dial velocity in one of the outliers would need to ob- range. Relaxing those assumptions would predict shorter served before its possible binary status can be ruled out, orbital timescales and more rapid radial velocity varia- we have constructed a simple model of each outlier as a tions. Thus, the timescales we derive for the completion binary system. The binary parameters actually allowed of a full orbit represent upper limits – changes in radial for each source by the observed spectra depend on the velocity may happen on shorter timescales, but should not occur on longer timescales unless the system has a specific Teff , veiling, radius, and extinction (and spatial structure thereof) associated with each source. In gen- primary with Teff greater than that detected by D05. eral, the observed veiling and lack of significant HI Br The resultant maximum orbital period for each outlier is γ absorption rule out the presence of hot, massive early shown in column 3 of Table 2, with corresponding binary type companions. Accordingly, to provide upper limits separations given in column 4. Typical orbital timescales on the orbital timescales these possible binaries should for systems under these assumptions are on the order of vary on, we will model each system as consisting of equal several years, with typical binary separations of 2-5 AU. Protostellar Radial Velocities 7

5. DISCUSSION with a 1.0 M⊙ primary as a function of period (from 1 to The radial velocity measurements analyzed here repre- 1000 days) and secondary mass. Their typical detection sent the largest sample of radial velocity measurements efficiency is about 60%, though dropping significantly to- for Class I/flat spectrum sources yet achieved. These wards longer periods. Applying a similar correction to measurements allow investigation of a number of dynam- the number of radial velocity outliers seen here would im- ical aspects of low mass star formation. ply a true companion star fraction of ∼ 22%. This would As shown in Section 3, the observed protostellar radial be roughly consistent with the companion star fraction velocity dispersion (σ = 2.59 km sec−1) is not signifi- measured for T Tauri stars over the same range of separa- cantly broader than would be expected given the obser- tions (see Figure 6 of Melo 2003, interpolating over the vational limits of this study. These observations therefore gap in data for systems with periods between 100 and place constraints on the width of the protostellar veloc- 10000 days ). However, as this study is based on single ity dispersion to lie below the level of our observational epoch observations, the completeness fraction is likely uncertainties, or ∼ 2.5 km sec−1. Velocity dispersions significantly lower than 60%, implying an even higher of CO gas (as measured using gaussian fits) in these star ‘true’ companion star fraction, in excess of that observed formation regions are typically ∼ 1.4 km sec−1; the upper for T Tauri stars at the same separations. Given the limit placed on the protostellar velocity dispersion in this good agreement between the T Tauri and protostellar study is consistent with the local CO gas and protostars companion star fractions at spatially resolvable separa- being dynamically linked during the Class I/flat spec- tions, such a difference in multiplicity characteristics at trum stage, with little dynamical evolution from stellar close separations would be somewhat surprising. As a interactions. This upper limit does not, however, rule result, it seems unlikely that all 4 radial velocity outliers out possible dynamical effects which could inflate the are short period protostellar binaries. protostellar velocity dispersion above that of the local The four radial velocity outliers can also be inter- CO gas, but below the 2.5 km sec−1 level. Future ob- preted as single stars with large intrinsic space veloci- servations of protostars producing spectra with higher ties. Such objects have presumably been ejected from signal-to-noise ratios or more simultaneous wavelength their initial formation sites by dynamical interactions coverage than obtained by D05 may allow for more ac- with other cluster members, as suggested for PV Ceph by curately measured velocities via cross correlation with Goodman & Arce (2004). The blueward velocity shift of spectral templates, placing tighter constraints on the dy- all 4 outliers could also be expected as a result of an ejec- tion model, as introduced in Section 2.2. However, the namical state of protostars. Radial velocity uncertainties −1 introduced by relatively large rotational velocities, how- high velocities reported here (7.5 km sec and larger) ever, may prove difficult to overcome. are unusual even within the context of ejection models, Four of the 31 (12.9%) Class I/flat spectrum sources as simulations of stellar encounters in a cluster environ- appear to have radial velocities more than 3 σ away ment (Delgado-Donate et al. 2003; Bate et al. 2003) pre- from the local cloud velocity. However, given these dict one dimensional velocity dispersions on the order of a few kilometers per second, and maximum ejection ve- high vsinis measured for these sources and their non- −1 discrepant radial velocities as derived from optical spec- locities of ∼ 5 km sec . The detection of spectroscopic tra (White & Hillenbrand 2004), followup spectroscopy accretion signatures also suggest the mass reservoir of to confirm these radial velocities should be taken before these protostars have not been disrupted recently by dy- these results are overinterpreted. If real, these detections namical interactions. We therefore view interpretation may present a lower limit to the close binary fraction of these high velocity outliers as a population of ejected cluster members as unlikely. of protostars. Haisch et al. (2004) and Duchˆene et al. −1 (2004) find companion star fractions of Class I/flat spec- However, our upper limit of ∼ 2.5 km sec on the trum objects similar to that of more evolved T Tauri one dimensional radial velocity dispersion of protostars stars (∼ 20 − 25%) in the star formation regions studied is not inconsistent with predictions of models of star for- here. The photometric nature of their study makes them mation which include lower velocity dynamical effects. most sensitive to companions with differences in K band As noted earlier, dynamical simulations of clustered star formation find 1-D radial velocity dispersions of 1-2 km less than 4 to 6 magnitudes and at −1 orbital distances from 300-2000 AU. A radial velocity sec , comparable to the level of observational uncertain- survey such as this is biased towards finding binaries at ties in this study. Additionally, halos of X-ray sources much closer radii: an equal mass binary system with 0.5 have been detected near sites of ongoing star forma- tion in ROSAT X-ray surveys. These halos typically M⊙ components in an edge-on, circular orbit with an or- bital velocity of 7.5 km sec−1 (the 3 σ limit of this study) extend to ∼ 5-10 degrees away from the center of the will have a separation of 3.9 AU and a period of 7.8 years. star formation region (Feigelson 1996; Neuhaeuser et al. Protostars with larger primary masses will have greater 1997; Wichmann et al. 2000). The X-ray sources popu- separations for similar orbital velocities. Thus, the 12.9% lating these halos possess Li absorption features, lead- of stars detected in this work as radial velocity outliers ing some to identify them as a dispersed population of could represent a single epoch lower limit to the number Weak lined T Tauri Stars, while others find a lack of of protostars with close companions within ∼ 10 AU. M type members of this population (which deplete their The companion star fraction lower limit presented lithium on faster timescales than G & K stars) indicative above, however, is uncorrected for incompleteness. As of an older population unassociated with ongoing star part of a multi-epoch radial velocity survey for short formation (Briceno et al. 1997; Brice˜no et al. 1999). We period (P < 100 days) T Tauri multiple systems, Melo find that the formation of dispersed halos of young X-ray (2003) calculates incompleteness corrections for binaries sources could result naturally from the expansion of clus- ters whose radial velocity dispersions lie somewhat below 8 Covey et al. the upper limits presented here and by Hartmann et al. • We detect 4 of the 31 objects (12.9%) studied here (1986) for T Tauri stars. For instance, at the distance of as 3 σ outliers from the local CO velocity. Further Taurus (140 pc; Elias 1978) a star moving 1 km sec−1 in observations are needed to confirm these supris- the plane of the sky can travel 7◦.5 in 18 Myrs, consistent ing velocities, which if true, imply either a large with the size and estimated ages of X-ray halos. Thus, (< 25%) close (r < 10 AUs) binary fraction, or a definitive analysis of the ability of pre-main sequence a significant level of dynamical ejections of young stars to produce diffuse halos of X-ray sources will require stars whose accretion reservoirs are nonetheless rel- stronger constraints on the radial velocity distribution of atively undisturbed. The 4 outliers detected here star forming regions than are currently available. do have negative measured radial velocities, con- sistent with the blueward bias expected for ejected 6. CONCLUSIONS objects. Our analysis of the radial velocities of Class I and flat spectrum protostars from three nearby star formation regions results in the following conclusions:

• The one dimensional radial velocity dispersion of Class I/flat spectrum protostars is measured to be 7. ACKNOWLEDGEMENTS ≤2.5 km sec−1. The authors wish to recognize and acknowledge the • The protostellar velocity dispersion upper limit very significant cultural role and reverence that the sum- presented here is consistent with the measured ve- mit of Mauna Kea has always had within the indigenous locity dispersions of both CO gas and more evolved Hawaiian community. We are most fortunate to have the T Tauri stars in the same star formation regions. opportunity to conduct observations from this mountain. This is consistent with the dynamics of the CO gas, The authors would like to thank George Herbig for an protostars and T Tauri stars being dominated by extremely rapid referee report; K.R.C is grateful to Anil the form of the large scale gravitational potential. Seth for useful discussions whose insights have improved However, the 2.5 km sec−1 protostellar radial ve- this work. All data have been reduced using IRAF; IRAF locity dispersion upper limit measured here is not is distributed by the National Optical Astronomy Obser- in itself inconsistent with models of star formation vatories, which are operated by the Association of Uni- in which dynamic stellar interactions play a signif- versities for Research in Astronomy, Inc., under cooper- icant role. ative agreement with the National Science Foundation. This research has made use of NASA’s Astrophysics Data • No large scale bias towards negative radial veloci- System Bibliographic Services, the SIMBAD database, ties is detected in this sample, as may be expected operated at CDS, Strasbourg, France, and the VizieR for stars with ejection velocities carrying them to- database of astronomical catalogues (Ochsenbein et al. ward Earth and to areas of lower extinction. The 2000). K.R.C gratefully acknowledges the NASA Grad- lower extinctions seen by these members would bias uate Student Researchers Program (NGT 2-52294) and magnitude limited samples towards their inclusion, T. Greene acknowledges NASA’s Origins of Solar Sys- thus translating into a stellar median velocity offset tems Program for valuable support that enabled this from that of the local CO gas. work (Task 21-344-37-22-11).

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