5. Implications of Gould Disk scenario
In this section we relate the proposed geometrical arrangement of the Gould Disk excess stars to previous observations of the GB stellar population and show that our concept gives new insight for the interpretation of major parts of the large-scale spatial distribution of young X-ray active stars in the RASS. We will also point out the importance of this excess population for the recent star formation history in the solar neighbourhood.
5.1. 3D spatial distribution
We start with the discussion of the three dimensional spatial distribution of excess stars. Using distance, galactic longitude and latitude, we computed the three-dimensional position of each star in a coordinate system centered on the Sun with the X-axis pointing towards the galactic center, the Y-axis pointing towards l = and the Z-axis pointing towards the north galactic pole. We divide our RasTyc sample into two groups depending on whether LX is above or below erg s-1, and show projections of the positions of the stars in a plane perpendicular to the galactic plane and containing the l and directions. The best contrast between the GB and galactic plane populations is obtained for the projection along l = , which is approximatively perpendicular to the nodal line of the GB. In Fig. 3 we show the projection for the X-ray bright stars, i.e., LX erg s-1 (left panel) and the X-ray faint ones, i.e., LX erg s-1 (right panel).
The X-ray bright component is not uniformly distributed over its volume but shows a striking inclined bar structure when projected perpendicular to the nodal line (upper left panel). Projection along the nodal line (lower left panel) shows a density enhancement mainly spread over the lower left part of the graph but not distributed on a ring as one might expect for the belt scenario. On the other hand, X-ray faint stars are distributed quite uniformly within 80 pc from the Sun independent of which projection is chosen. The inclined bar structure in our Fig. 3 upper left panel is very similar the one shown in Westin (1985) or Gaustad & Van Buren (1993), who plot the distribution of O -A0 and O6 - B9.5 in the same projection. Their sample stars are known to be young because of the small life time of early-type stars. We are dealing with a completely different but complementary population of late-type stars (F G (K)). Our proxy indicator for youth is their strong X-ray activity. The coincidence of the spatial inclinations strongly suggests a close, generic relation of both populations. Ages of the order of 30 to 80 Myrs, as proposed for the age of the GB (Eggen 1961, Lesh 1972, Westin 1985, Fresneau et al. 1996), are indeed compatible with the observed X-ray luminosities. We also note that if the F and G type stars are in the above age range, they would already appear on the ZAMS in the Hertzsprung-Russell diagram (HRD), because they already finished their PMS phase. As a consequence, a selection of GB members based on Hipparcos parallaxes and their location in the PMS region of the HRD seems feasible only for a few, very late type (K and M) stars in the sample.
5.2. Relation to the distributed young RASS population
This subsection relates the Gould Disk picture to the spectroscopic follow-up observations of optical counterparts to RASS sources in regions of the sky related to the appearance of the GB. In general, the purpose of the identification programs was to obtain a reliable classification of previously unknown RASS sources. In the present context, the classification criterion of interest to us is the observed strength of the Lithium I equivalent width in the optical spectra in order to obtain an independent age indicator. Although a unique Lithium-age relation has not been convincingly demonstrated because of the strong influence of stellar mass and rotation (e.g. Soderblom 1996), Li I absorption strengths above the levels of well-studied, coeval open clusters like the Pleiades, Per or IC2602 (resp. ages 70Myr, 50Myr, 30Myr) in conjunction with other signs of high chromospheric or coronal activity provide useful constraints on their youth (Pasquini et al. 1994, Guillout 1996, Neuhäuser et al. 1997).
As the specific objectives for individual studies were different (e.g. complete identifications of flux-limited samples for statistical purposes, or discovery of young, possibly even PMS stars in samples biased towards high X-ray activity), the stars were selected rather inhomogeneously and statistical properties like frequency of Li-rich stars cannot be easily compared quantitatively. Nevertheless, the wide spatial sampling in the sky does allow to draw a number of conclusions on the possible presence of a young, X-ray active "excess" population in individual fields. For a better orientation, the location of the four regions considered with respect to the assumed geometry of the Gould Disk are indicated in Fig. 2.
5.2.1. Cygnus (l = )
In the course of deriving the fraction of the various populations of X-ray emitters encountered at low galactic latitude in soft X-ray surveys and to test the predictions of the Besancon X-ray stellar population model (Guillout et al. 1996a), Motch et al. (1997) present results of an identification program of 128 RASS sources in an 64.5 deg2 large field in the galactic plane ("Cygnus-field" centered at l = and b = ) and show that the density and distribution in flux and spectral type of the dominant population of active stars is in fact consistent with current stellar population models and age dependent X-ray luminosity functions. The log(NS)-log(S) relation is in full agreement with the predicted X-ray number counts assuming a constant star formation rate and a canonical slope for the initial mass function. No evidence for an additional, young "excess" population is evident from the data.
How does this result relate to the above depicted Gould Disk scenario? First we note, that the projected central location of the GB at that galactic longitude (l = ) is approximately located at a latitude of . As the GB is expected to have a scale height comparable to that in the other quadrants ( ), the "Cygnus-field" should significantly overlap with the projected GB region. As the X-ray flux- limit of the Motch et al. (1997) study is similar to that considered here (Sthr = 0.03 cts s- 1, but not limited in optical magnitudes), we expect to sample similar distances 300 pc as indicated in Fig. 2. If the Gould Disk is indeed devoid of stars within 300 pc in this direction as suggested in the sketch Fig. 2, we would indeed not expect to find an excess of particularly young, X-ray active GB members at that X-ray flux-limit. We conclude that the absence of any observed excess population with respect to predictions of standard galactic model in this field, is fully consistent with the assumption of a Gould Disk with an inner radius further than 300 pc in the direction of l = .
5.2.2. Taurus (l = )
The discovery of many (more than 50) new PMS stars in and around the Taurus-Auriga SFR has been reported by Wichmann et al. (1996), Magazzu et al. (1997) and Neuhäuser et al. (1997). In these cases, the observed samples are mainly drawn from the cross-correlation of the RASS with the Hubble Guide Star Catalog (GSC, Lasker et al. 1990), and therefore reach fainter stars down to 15. In general, the large Lithium equivalent widths for these young stars, especially for the late-type K and M stars, favour the interpretation of them being PMS with ages around 10 Myrs and members of the SFR. But, lacking precise distances, these stars could, in principle, also be located in the foreground of the Taurus-Auriga clouds. Recently, Neuhäuser & Brandner (1998) reported four of those stars in the Hipparcos catalog with distances in the range of 100-150 pc, therefore suggesting that these stars are member of Taurus rather than foreground stars. The geometrical situation regarding the GB is complicated in this region, as the GB central plane crosses the southern parts of the Taurus SFR at l and b . We restrict the following discussion to the field south of Taurus, including a strip perpendicular to the galactic plane along l = , which was chosen to search for the southernmost TTS of Taurus. Neglecting the contamination with TTS located in the nearby -Ori region they find 10% Li-rich stars in their activity selected sample which they conclude to be not older than the 30 Myrs old cluster IC 2602. The spatial distribution of their Li-rich stars suggests an association with the GB, however, alternative interpretations for their presence cannot be excluded. In summary, the Neuhäuser et al. (1997) data do not conclusively prove the existence of a young excess population that can be identified with the GB.
In addition, a complete identification of RASS sources in a 70 deg2 large field south of Taurus (l = and b = ) did not provide any evidence for an excess population at the considered flux-threshold of Sthr = 0.03 cts s- 1, but rather demonstrated (like in the "Cygnus-field") the consistency of the RASS population with the X-ray number count models in that direction of the sky (Guillout et al. 1996b, Guillout 1996). According to Fig. 2 we would actually not expect a large contribution of GB members in the above mentioned samples, as the GB is expected to be located in the background of the Taurus clouds, and not detectable in the RASS with a distance in excess of 300 pc.
5.2.3. Orion (l = )
The situation is considerably changing with increasing galactic longitude towards the Orion SFR ( ). Sterzik et al. (1998) report on an effort to identify RASS sources in a 10 degrees wide strip centered on l = crossing the galactic plane perpendicular to it. Whereas one sub-region located in the galactic plane ( ) is fully consistent with expectations from the X-ray number count model by Guillout et al. (1996a), the southern field located on the GB ( ) shows a significant excess of RASS sources at a given flux-threshold when compared to model predictions. Optical follow-up observation using high-resolution spectroscopy confirm that a significant fraction of the counterparts is Li-rich. Interestingly, their spectral types tend to be G or earlier, indicating that this population is not nearby but somewhat more distant. The geometry sketched in Fig. 2 suggests that GB members could in fact be detected in the foreground of the Orion SFR within distances of 200-300 pc, i.e., in reach of the RASS flux-threshold.
5.2.4. Lupus (l = )
Wichmann et al. (1997) carried out an identification program of X-ray active stars in an area adjacent the Lupus SFR within a 10 degrees wide strip centered on l = crossing the galactic plane, but perpendicular to it. The distribution of their 48 Li-rich stars (among a total sample size of 160 objects) strongly peaks at , with a gradient towards the galactic plane. Since this location is precisely at the intersection of their study region with the GB they interpret their sample of Li-rich stars as low-mass members of the GB with ages not exceeding 50-60 Myrs. In contrast to the field near Orion, in Lupus the spectral type distribution of Li-rich stars is rather skewed towards objects with spectral types later than K0, reaching down to M1.5. This means that these objects are likely to be systematically closer than those from the Orion strip as both samples have the same selection biases.
According to Fig. 2 the Lupus SFR is located at the near side of the GB and should in fact allow a rather complete sampling of GB members in the foreground of the SFR itself. The spectroscopic data are consistent with the detection of a foreground population in this particular sky direction.
5.3. Star formation history in the solar neighbourhood
Next we want to put our findings in the more general context of the local star formation history. Suppose that the distribution of low-mass young stars qualitatively agrees with the sketch in Fig. 2. Depending on the exact distance of the Sun towards the nearest GB members (note that the inner radius of the assumed Gould Disk is not well constrained by the current data set), the solar neighbourhood should be (at least in part) embedded in a halo of young GB members. Is there any evidence for low-mass stars in the solar neighbourhood with ages considerably younger than those of the Pleiades cluster and in excess of standard galactic models?
There are several independent indications of very young and possibly even PMS stars in the nearby field. Eggen (1995) proposed that isolated PMS members of the Pleiades Supercluster, which contains several smaller open clusters sharing the same space velocity, are among the nearby stars from a systematic study of the kinematics of the solar neighborhood. On the basis of similar kinematic arguments, Jeffries (1995) suggests an active stellar population, the so-called Local Association, containing a significant proportion of very young late-type stars. A few isolated PMS stars or candidates have been discovered on the basis of IRAS data (de la Reza et al. 1989), or on the basis of large EUV (Jeffries et al. 1994, 1996), or X-ray activity as observed with the Einstein Observatory (Favata et al. 1995, 1997) or the EXOSAT satellite (Tagliaferri et al. 1994).
Chromospheric activity is also often used to infer the evolutionary status of cool stars. The Ca II H&K flux levels decrease gradually with age (see e.g. Skumanich 1972) with an unclear functional form. Since the Ca II H&K flux levels are closely related to magnetic and coronal activity, they are an independent proxy of age. A striking result of an extensive survey of stars in the solar neighbourhood is the bimodal frequency distribution of the flux ratio in the Ca II H&K emission cores to that of the nearby continuum (Vaughan & Preston 1980, Henry et al. 1996). The apparent lack of stars with intermediate activity suggests an overabundance of young and active stars in the solar neighbourhood; this interpretation is, however, not undisputed, because different, equally well or poorly justified assumptions on the activity-age relation can mimic the impression of a (statistically actually insignificant) gap (Hartmann et al. 1984, Barry 1988, Soderblom et al. 1991).
Recently Sterzik & Schmitt (1997) analysed the coronal activity distribution as expressed with for a sample of 1 238 cool stars within 25 pc around the Sun. Their X-ray data confirm the existence of a statistically significant gap for intermediate active stars in the color range . They show that the coronal most active objects also exhibit the largest Li I equivalent widths and infer that young, late-type stars do indeed exist in the solar neighbourhood.
Thus different, independent pieces of evidence suggest the presence of a particularly young population of cool stars in the solar neighbourhood. It is less clear, however, whether these stars form an excess population that cannot be explained by a standard galactic modeling and that require a recent episode of star formation. However, we note that nearby members of the GB could well be immersed in the sample of young, nearby stars.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998