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while young , stars are rapidly rotating which causes  activity " manifest as chromospheric and coronal emission . the fractional flux emitted in the ca  ii @xmath4 and @xmath5 line cores relative to the stellar bolometric flux
is known as @xmath2 ; see e.g. noyes et al .
( 1984 ) for the formalism .
empirical rotation - activity correlation is illustrated in e.g. white et al .
( 2007 ) and for our sample in hillenbrand et al .
activity increases with increasing rotation at slow rotation speeds ( @xmath610 - 15 km / s ) until becoming  saturated " at higher values of v sin_i_.        rapid rotation also broadens spectral absorption lines , making precision radial velocities harder to measure .
separately , there is  jitter " in observed radial velocities as the temperature structure of the projected stellar disk changes on time scales associated with photospheric granulation / convection , rotation periods and activity cycles .
figure 1 illustrates the relations among rv precision and rotation - activity - jitter for our sample stars .
-0.25truein   values for 1638 fgk stars as reported in isaacson & fischer ( 2010 ) , representing the overall cps sample ( open histogram ) , compared to that of the 171 stars discussed here ( shaded histogram ) , nearly all of which are included in the open histogram as well .
histograms are normalized by the total number of stars in each sample .
activity level increases to the left .
, title="fig:",scaledwidth=65.0% ]    within the context of the broader california planet search program ( e.g. wright et al .
2012 ) , a sample of @xmath0150 solar - neighborhood stars having estimated masses 0.8 - 1.2 @xmath7 , and ages younger than the sun have been monitored .
the program began in 2002 , with some stars added in 2004 and others in 2008 .
the targets were drawn from stars for which detailed information on the presence and properties of dusty disks is available . included
are stars from the feps ( meyer et al .
2006 ) and other debris disk surveys , where dust presence indicates the existence of already formed planets , and selected very young stars associated with regions of recent star formation , possibly still in the process of forming planets .
our sample stars are young and active relative to the typically quieter stars that are the focus of much of the ongoing radial velocity planet search work ; see figure 2 .
as illustrated in figure 3 , there is a direct correlation between the rms scatter in the observed radial velocity time series and the mean chromospheric activity level .
a linear fit suggests activity - induced radial velocity jitter of 195 m / s or more for very high activity levels , around log @xmath8 , and just 3 m / s ( near the 3@xmath9 measurement errors ) for lower activity levels , around log @xmath10 .    , with stellar activity level , @xmath2 .
dashed line is a linear fit to our data ( blue points ) after three iterations of rejecting 3@xmath9 outliers ( black points ) . for comparison ,
also shown are the rms of measured radial velocities for ( red points ) the 9 t tauri stars studied by crockett et al .
( 2012 ) at an arbitrarily chosen activity level of log @xmath11 , and the median for hyades stars reported by paulson et al .
( 2004 ) , adopting the median log @xmath12 from mamajek & hillenbrand ( 2008 ) ; at intermediate activity levels data ( magenta points ) from lagrange et al .
( 2013 ) are shown .
, scaledwidth=60.0% ]
our young stars are more active than those investigated in previous pursuit of jitter - activity scaling .
figure 4 illustrates how various of these relationships  all derived using inactive star samples  would propagate to the more active stars considered here .
our relation is much steeper at the active end .
-0.6truein   -0.9truein
the empirically determined radial velocity variation , @xmath1 , as a function of activity level , @xmath2 , can be turned in to a minimum detectable companion mass .
we utilize the formalism given by narayan et al .
( 2005 ) , following cumming ( 2004 ) and nelson & angel ( 1998 ) .    for active stars , when the number of observations exceeds @xmath1320
, meaningful limits can begin to be placed on planetary mass companions having orbits shorter than the duration of the time series .
currently we reach sub - jupiter mass sensitivity for roughly few au orbits , as illustrated in figure 5 .
star , as a function of stellar activity .
activity is translated to radial velocity noise using our empirical relationship , and then to mass based on simulations .
the plot panels are for different numbers of radial velocity measurements ( nobs ) and within each plot , the solid , dashed , and dotted - dashed lines correspond to planet periods of 1 , 10 , and 1000 days .
the horizontal dotted lines show the masses of jupiter ( in blue ) , neptune , and earth .
, scaledwidth=102.0% ]
we report initial results from our long term search using precision radial velocities for planetary - mass companions located within a few au of stars younger than the sun . based on a sample of @xmath0150 stars , 
 we define a floor in the radial velocity scatter , @xmath1 , as a function of the chromospheric activity level @xmath2 . 
 this lower bound to the jitter , which increases with increasing stellar activity , sets the minimum planet mass that could be detected . adopting a median activity - age relationship reveals the astrophysical limits to planet masses discernable via radial velocity monitoring , as a function of stellar age . 
 considering solar - mass primaries having the mean jitter - activity level , when they are younger than 100 / 300 / 1000 myr , the stochastic jitter component in radial velocity measurements restricts detectable companion masses to @xmath0 0.3 / 0.2 / 0.1 @xmath3 . 
 these numbers require a large number  several tens  of radial velocity observations taken over a time frame longer than the orbital period . 
 lower companion mass limits can be achieved for stars with less than the mean jitter and/or with an increased number of observations .