THE INTERSTELLAR MEDIUM AND STAR FORMATION

I. A Complex Region in Cygnus The H- images used in this exercise were obtained by Drs. B. Dennison (UNCA), J. Simonetti (VirginiaTech), G. Topasna (VMI), P. Nelson (Roanoke College), C. Hall (UMD), C. Kelleher (Washington U.St. Louis); and students J. Ruffner and M. Wallace.

To view and analyze this image you will use the professional astronomy imaging software SAOImage DS9. Begin by making sure that the image file we plan to analyze first (cyg.fit) is available on your desktop. Next, start DS9 and load the file (File > Open). Before examining the image in detail, you may want to adjust the contrast. To do this, first set your editing mode to colorbar by selecting (Edit > Colorbar). Then, click on the image, hold down the right mouse button, and move the cursor vertically and horizontally. This will simultaneously adjust the brightness and the contrast of the image. You may also want to experiment with changing the brightness scale of the image from its default setting (linear) to another setting, e.g., (Scale > Square Root). A satisfactory result should give you a good view of most parts of the image.

Further manual adjustment of the brightness and contrast will probably be desirable. You can adjust the minimum and maximum brightness limits of the image by going to (Scale > Scale Parameters). This will return a plot of the total number of pixels in the image as a function of brightness. Dragging the red and green bars will change the minimum and maximum brightness used for scaling the image contrast. As you experiment with moving these limits, you can see the results: reducing the maximum value saturates the bright features, but renders fainter structures visible. Increasing the minimum value tends to reduce the background to a suitably dark level, but if set too high, then faint structure in the image is lost. In most cases, the minimum value should not be increased drastically; often a value between 1000 and 3000 works well. Recommended values to try for minimum and maximum are given in the table below for the images used in this exercise. These are recommendations only, and you can experiment with other settings and fine-tune (moving the mouse over the image with right-click button held) to bring out more details.

IMAGE	Recommended Minimum Contrast Setting	Recommended Maximum Contrast Setting
cyg.fit	3300	6000
cyg_ha.fit	2300	10000
coo_ha.fit	2000	4100
coo.fit	3300	7200
rose_ha.fit	1000	3400 (12000)
rose.fit	2000	47000
w4_ha.fit	2200	3500 (10300)
cas_ha.fit	2200	4300
tau_ha.fit	2200	3900
fil_ha.fit	2000	5000

With minimum--maximum settings that let you see the many faint stars in the Cygnus image, note the irregularity in their distribution over the field of view. In the space below (or as a separate attachment with your submission) sketch what you see and indicate approximately where dark regions, which seem to have noticeably fewer stars, are located in the image:

Question: What causes the dark regions in the image?

A zoomed-in view of the cursor position is shown in the upper-right of the DS9 window. To zoom in and out, use the roll bar on the mouse or use the functions in the Zoom menu (Zoom > Zoom In / Out). Once you have zoomed-in for a closer look at the image, you can drag the blue box in the upper right-hand panel to the portion of the image you wish to view.

Next, load the H- image of this same field in a new frame, while keeping the continuum image open and available in a different frame. To do this select Frame > New Frame, then File > Open to load the next image: (cyg_ha.fit). You will now be able to move between the two loaded images at any time by clicking on the frame button and next clicking on prev or next. Set the minimum--maximum values and compare the H-alpha image with the continuum image of precisely the same region.

When comparing different images of the same object, it can be useful to blink between the frames frame -> next / prev. If the images do not seem to be aligned (e.g., if one of the images has been set to a different zoom or centering), you can re-align the frames with the following steps in the Frame dropdown menu: (Frame > Match > Frame > WCS).

Question: Comparing these two observations of Cygnus, can you see what a difference it makes when we observe this same region in H-? What does the H- exposure show us that was essentially invisible in the continuum image? Explain.

The bright glowing regions are called emission nebulae -- clouds of hydrogen ionized by newly formed hot, young stars. In the space below (or as a separate attachment with your submission) sketch the approximate outlines of the brightest emission nebulae in the H- image.

You will note that the H- image has dark regions as well. Notice that some of these correspond with dark regions in the continuum image. Indicate dark regions common to both images in your sketch above.

The bright nebula near the pixel coordinates (385,125) in the H- image is called the North America Nebula (and catalogued as NGC 7000). The information in the upper left of the DS9 window gives the pixel coordinates as you move the cursor around with the mouse. A zoomed-in view of the cursor position is shown in the upper-right of the DS9 window. Obtain a set of pixel coordinates on either side of the nebula, and record those coordinates below:

x1,y1= x2,y2=

Now use the distance formula,

d= (x2-x1)2+(y2-y1)2

to determine the separation between these two points in pixel units:

Convert this distance into an angle, A, in degrees, using the fact that the camera image scale is 0.0267 degrees / pixel. Thus, the angle in degrees is:

A degrees= 0.0267 degreespixel* d

The result you just calculated is a measure of the approximate width of the North America Nebula in degrees. Now, the Full Moon is about half a degree across, so the North America Nebula is actually pretty big in the sky, considerably larger than the Full Moon! Its not readily apparent to the unaided eye because it is quite faint, not because it appears small.

Before proceeding to the next pair of images, you should close the two you have open. Do this by clicking frame followed by delete, once for each image.

The Cocoon Nebula -- Stars form in cold, dense clouds. Soon after formation, stars of type O and B ionize their surroundings, but much of the original star formation cloud remains. Often these remnants of star formation have a linear appearance extending away from the newly formed star(s). Load the H- image of the Cocoon Nebula (coo_ha.fit) into a new frame in DS9.

After resetting the minimum and maximum contrast values (as per the table on the first page), you'll notice a small bright object at the end of a linear dark structure near the pixel coordinates (150, 190). The object is so small in appearance that it might just be a star. If it is a star it should appear in the continuum image as well. On the other hand, if it's a small region of ionized gas (emitting H- light) then it should not be obvious in the continuum image.

Now, bring up the continuum image (coo.fit) in a new frame while keeping the H- image open in another frame. (Remember to set the minimum--maximum values in the continuum image after pulling it up in a separate frame. You may also want to re-align the images before blinking them for comparison. Frame > Match > Frame > WCS) What do you conclude about the nature of this object (i.e. is it a small emission nebula or a star)?

Although the Cocoon Nebula appears small, its angular diameter can be measured. Recall that you can zoom-in and then re-center the view by dragging the blue box in the window in the upper right. Obtain the coordinates of points on opposite sides of the nebula, apply the distance formula, and convert your result into degrees:

x1,y1= x2,y2=

d =

A =