The interstellar medium towards the Ara OB1 region.

The Ara OB1 association is located in the Carina-Sagittarius spiral arm ($\it l\sim 337^{\circ}$, $\it b\sim-1.5^{\circ}$) and contains several clusters of hot young stars that include NGC 6167, 6193 and 6204, as well as the molecular cloud/HII region complexes of RCW 108 and 107. With a distance of $\sim$ 1320 pc (Herbst $\&$ Havlen, 1977) this complex lies slightly foreground and adjacent to the larger Sco OB1 association. The interstellar sight-line to this region of the Galaxy is known to be complex, and it contains several gas and dust clouds with velocities covering the $-50$ to $+10$ km s^-1 range (Arnal, May $\&$ Romero, 2003). A constant reddening of $\sim$ 0.2 mag has been found in this direction reaching to a distance of $\sim$ 1 kpc (Herbst $\&$ Havlen, 1977), with the majority of this interstellar dust residing within 200 pc of the Sun. Beyond this local extinction there appears to be a clear interstellar region, $\sim$ 1 kpc wide, which is the well-known ‘inter-arm’ gap that separates the local spiral arm from the adjacent Carina-Sagittarius arm.

The young stars in Ara OB1 are known to have a profound effect on the surrounding interstellar medium (ISM), with their out-flowing stellar winds imparting pressure to form expanding shells of gas. Many such interstellar features have been discovered from HI and H$\alpha$ observations of the area. For example, Arnal et al. (1987) have found an expanding HI emitting shell centered on NGC 6167 with a kinematic distance of 1400$\pm 200$ pc and an outer radius of $\sim 50$ pc. Subsequent HI observations of this region by Rizzo $\&$ Bajaja (1994) have further revealed a pervasive two-shell structure with gas cloud LSR velocities centered at $-16$ km s^-1 and $-34$ km s^-1, each with respective kinematic distances of 1.4 and 2.5 kpc. The nearer gas shell was speculated to be formed as a result of stellar winds (and/or supernova explosions), which sequentially triggered the star formation process in Ara OB1 after running into a nearby molecular cloud less than $2\times 10^{6}$ years ago (Waldhausen, Martinez $\&$ Feinstein, 1999). H$\alpha$ observations of this region by Georgelin et al. (1996) have also revealed similarly pervasive emission features with LSR velocities of $-28$ km s^-1 and $-39$ km s^-1.

More recent HI observations for the Southern Galactic Plane Survey by McClure-Griffiths et al. (2002) have revealed a large 10^∘ angular diameter shell, named GSH 337+00$-$05, surrounding this entire region that is centered approximately on the galactic plane. The authors derive a systemic LSR velocity of $-5$ km s^-1, an expansion velocity of 9 km s^-1 and an energy of $\sim 1.6\times 10^{44}$ J for this large interstellar feature. They place the distance of this gas shell at 570$\pm 900$ pc, which would imply that it lies considerably in front of the Ara OB1 cluster, although the expansion energy of the shell is consistent with its formation by a young association of hot stars (such as those of Ara OB1). Interstellar NaI and CaII absorption observations of the sight-lines towards early-type stars in both Sco OB1 ($\it l=343^{\circ}$, $\it b=+1^{\circ}$) by Crawford et al. (1989) and in Ara OB1 by Whiteoak $\&$ Gardner (1985) have revealed several gas cloud components which show several well-defined groupings with LSR velocities in the $-15$ to $-50$ km s^-1 range, similar to that found from the HI data. However, these negative velocity absorption components have been associated with interstellar gas at a distance $>$ 1 kpc that has supposedly been disrupted by the stellar winds of the two aforementioned OB associations. Thus, the interpretations of the radio HI emission and the ground based absorption data are presently in disagreement.

In this paper we present new high resolution ($R=70\,000$ or velocity resolution $\sim 4$ km s^-1) NaI and CaII absorption measurements towards 14 early-type stars located in the galactic direction towards Ara OB1. From these new, and previous, interstellar observations we can place a most probable distance of $\sim 530$ pc to the large GSH 337+00$-$05 HI shell, in contradiction to its physical association with the Ara OB1 cluster as argued previously by several authors. Instead, we argue that gas with absorption velocities in the range $-45$ km s${}^{-1}<V_{\mathrm{helio}}<-25$ km s^-1 is formed at a distance of $\sim 1$ kpc, and is far more likely to be linked to the interaction of the Ara OB1 stellar cluster with the ambient ISM.

Interstellar NaI D-line doublet (5890 Å) and CaII (3933 Å) absorption line measurements were obtained for the 14 early-type stars given in Table 1, which all lie within a radius of $\sim 7^{\circ}$ of the central position of the GSH 337+00$-$05 shell. We also list the galactic position, visual magnitude, spectral type, reddening (colour excess) and distances of these stars. The list (presented in increasing stellar distance) includes targets lying both in front of and beyond the nominal 1320 pc distance of the Ara OB1 association. In Figure 1 we show the position of all these (numbered) targets with respect to an HI image of the GSH 337+00$-$05 shell (McClure-Griffiths et al., 2002). The majority of our targets lie within $\pm 2^{\circ}$ of the galactic plane and most have sight-lines that are coincident with the brightest emission features of the $-5.5$ km s^-1 HI gas shell.

Well determined distances for the stars are an important parameter for this study, but unfortunately only one target (HD 147971) has an accurate Hipparcos trigonometric parallax measurement. For the remainder of the targets we present spectro-photometric distance estimates that have been derived using their spectral type, absolute magnitude and colour excess, as reported in the references listed in Table 1 for each star. The typical error for the majority of these derived distances is $\sim 25$ per cent (Hunter et al., 2006).

The spectroscopic observations were carried out during April 2006, June 2006 and July 2007 using the 1.0-m McLellan telescope at the Mt John Observatory in New Zealand. The HERCULES vacuum fibre-fed échelle spectrograph (Hearnshaw et al., 2002) was used with the 100 micron fibre and 50 micron slit to give a resolving power of $R=70\,000$ ($\sim 4$ km s^-1). The data were recorded with a Fairchild 4k $\times$ 4k CCD in a Spectral Instruments camera, and wavelength calibrations were obtained from thorium-argon hollow-cathode lamp spectra taken at the beginning and end of each night. The two spectral orders of interest were extracted from the raw CCD images using software routines written by J. Skuljan (University of Canterbury, Christchurch). Standard échelle reduction procedures were employed that included background and cosmic-ray subtraction, spectral order tracing and extraction, flat-fielding and wavelength calibration. For the case of the NaI D-line doublet order, each stellar spectrum was divided by that of the bright and nearby star $\alpha$ Eri in order to remove the narrow telluric water vapor lines that contaminate this wavelength region. The majority of the stellar spectra were well-exposed with typical S/N ratios in excess of 25:1 (less for the CaII data), and all velocities subsequently reported in this paper are in the heliocentric frame of reference (for comparison, $V_{\mathrm{L}SR}=V_{\mathrm{helio}}+3$ km s^-1).

The interstellar absorption profiles of the two NaI D-lines and the CaII K-line were fit using a dedicated software package described in Sfeir et al. (1999). This fit process entailed determining the local stellar continua for each absorption line using a multi-order polynomial fitting procedure. The resultant residual intensity profiles, whose equivalent widths are listed in Table 1 (with typical measurement errors of $\pm 10$ per cent), were then fit with multiple gas cloud components, each with a theoretical best-fit of cloud component velocity ($V$), cloud doppler velocity dispersion ($\it b$) and cloud component column density ($N$). The CaII absorption profiles were less saturated than the NaI lines and were fit prior to the NaI lines for each sight-line. The CaII model fit was then used as a basis for the subsequent fitting of the NaI D-lines. Each model fit procedure used the $\it minimum$ number of cloud components that were statistically significant using the criterion given in Vallerga et al. (1993). The two NaI D-lines were fit simultaneously in order to better constrain the model fits, whereas each CaII K-line was fit independently. The resultant absorption model fit parameters for each star are listed in Table 2, and the individual fits superimposed on the data points are shown in Figures 2–4. We have also limited the maximum gas cloud temperatures (i.e. the derived $\it b$-values) in the Na D-line fits to $15\,000$ K and $20\,000$ K for the CaII lines, in order to comply with the expected physical conditions in the diffuse ISM.

The low S/N ratio in the spectrum of HD 149019 at the CaII K-line means that the fit is probably over-interpreted. In that case knowledge of possible velocity components was gleaned from other distant stars and used to fit the line with the minimum number of components.

The line-fit procedure works well for unsaturated absorption components, but large errors can occur in the derived column densities for the central cores of highly saturated absorption lines. Unfortunately, since the majority of our targets lie along distant and high column density sight-lines, significant line saturation is present for cloud components with velocities in the $-20$ to $+10$ km s^-1 range. However, when components reside in the (red or blue) wings of the absorption profiles their column densities are better constrained. Thus, for most of the absorption profiles, only the very central components cannot be fit with certainty and these components are identified as such in Table 2. Much of the following analysis of our observations fortunately relies on the velocities, and not the column densities, of the numerous absorption components.

Whiteoak $\&$ Gardner (1985) presented high resolution (but low signal-to-noise) NaI absorption data towards 28 stars in the direction of Ara OB1 and have listed three pervasive absorption components at heliocentric velocities of $-37$, $-18$ and 0 km s^-1. Tentative evidence was presented that associated both of the negative velocity components with that of the Ara OB1 association, such that the gas clouds were placed within the Carina-Sagittarius arm at a distance of $\sim 1.3$ kpc. However, recent HI (McClure-Griffiths et al., 2002) and H$\alpha$ (Georgelin et al., 1996) measurements suggest a kinematic distance $>2.5$ kpc for these negative velocity gas components, thus apparently precluding a physical link with Ara OB1. Hence, in this present work, we wish better to identify the distances associated with the major interstellar gas clouds seen in this galactic direction by using new NaI and CaII absorption data, taken in the context of data recorded since 1985 at other wavelengths. First we follow the example of Whiteoak $\&$ Gardner (1985) by plotting the velocity extent of the absorption detected towards each target, with the stars ordered according to their distance, in Figure 5. This plot reveals four broadly distinct groupings of absorption components that cover the following velocity ranges towards the following targets: $-10$ km s${}^{-1}<V_{\mathrm{helio}}<+10\,$ km s^-1 (stars 1 $\&$ 2), $-20$ km s${}^{-1}<V_{\mathrm{helio}}<+20$ km s^-1 (stars 3, 4, 5 and 11), $-45$ km s${}^{-1}<V_{\mathrm{helio}}<-25$ km s^-1 (stars 6, 7, 8, 9, 12 and 13) and $-60$ km s${}^{-1}<V_{\mathrm{helio}}<-50$ km s^-1 (stars 10 and 14). We now discuss each of these absorption groupings (which we associated with separate interstellar gas clouds) in order to determine their respective distances and also to comment on the physical conditions of the ISM present over these four absorption velocity intervals.

We have made high resolution absorption measurements of the NaI and/or CaII lines towards 14 early-type stars in the galactic direction of the Ara OB1 cluster, which is thought to lie at a distance of $\sim 1320$ pc. The interstellar sight-line to this region is complex, consisting of many gas clouds with velocities ranging from $-60$ to $+20$ km s^-1. Previous radio emission and ground-based absorption measurements were in disagreement as to the distances to these clouds, particularly that of the large GSH 337+00$-$05 HI shell.

Our new measurements allow the observed line-profiles to be split into four distinct groupings of absorption velocity. For gas with absorption covering the velocity range $-10$ km s${}^{-1}<V_{\mathrm{helio}}<+10$ km s^-1, we can identify the observed absorption with local gas belonging to the Lupus-Norma interstellar cavity located a distance of between 100 and 485 pc. Gas with absorption velocities spanning the range $-20$ km s${}^{-1}<V_{\mathrm{helio}}<+20$ km s^-1 is only detected towards stars with distances of 570–800 pc. We identify the angularly large interstellar absorption feature at $V_{\mathrm{helio}}\sim-15$ km s^-1 with the expanding HI shell called GSH 337+00$-$05, which is now firmly placed at a distance of $\sim 530$ pc.

Gas with absorption velocities in the range $-45$  km s${}^{-1}<V_{\mathrm{helio}}<-25$ km s^-1 is only detected towards stars with distances $>1$ kpc, which places this gas at a similar distance to that of the Ara OB1 stellar cluster. The similarity between the radial velocity of the stellar cluster and that of the absorbing gas suggests that the cluster stars may be presently embedded within, and traveling through, this interstellar cloud complex. Absorption at velocities $<-50$ km s^-1 is seen towards only two stars, HD 154873 and HD 150958, both of which have distances $>1300$ pc. Although the observed NaI/CaII ratio for this component is consistent with the presence of interstellar (stellar wind-driven) shocks, we believe that it is unlikely that this highly negative velocity gas is physically associated with the Ara OB1 cluster itself, and its origin may be linked to the more distant (and larger) Sco OB1 cluster.

We gratefully acknowledge the staff at the Mt John Observatory for their help in making these observations possible. BYW acknowledges funding from NSF grant $\#$ AST-0507244.

Figure 1 was produced using data from the Southern Galactic Plane Survey (McClure-Griffiths et al., 2005).

This research has made use of the SIMBAD
database, operated at CDS, Strasbourg, France.

This research has made use of the VizieR database (Ochsenbein et al., 2000), operated at CDS, Strasbourg, France.